A Mardi Gras crowd of 70,000 packed into the Super Dome to hear two white comedians from New York sing the blues? It's enough to drive Muddy Waters to disco.

Nonetheless, John Belushi and Dan Aykroyd currently have the No. 1 record album in the country -- the Blues Brothers' "Briefcase Full of Blues." What started out as a musical comedy sketch on "Saturday Night Live" has already earned $2 million, according to Atlantic Records. Universal Studios will make it into a major motion picture, and a spokesman for the Blues Brothers says they turned down an offer of $60,000 for a one-night-stand during Mardi Gras in New Orleans.

"If they would take a few of these concert offers coming in, we'd all be living in big homes," the spokesman said. "But that's not where their heads are at."

It's not where their contracts are at either. Belushi and Aykroyd presently are commuting between "Saturday Night Live" and the set of Steven Spielberg's upcoming movie satire of World War II, "1941," in which they star. Consequently, they have little time to take their fictional duo -- "Joliet Jake" and "Elwood Blues" -- on the road, despite the briefcases full of money being offered by concert promoters.

"There have been some incredible offers," drawled Steve Cropper, a member of the Blues Brothers Band, "but our touring plans have to be limited because of John's and Danny's careers. We'll have to work around it."

During Cropper's 20-year career he earned a reputation as one of the best R & B session men in the business, playing behind the likes of Otis Redding, Wilson Pickett, the M.G.'s, Rufus and Carla Thomas. How does he feel about playing behind two funny men? "Well, I can't recall ever having a No. 1 album before," he said.

The success of the album apparently surprised all parties concerned. "It's probably the fastest-breaking record we've ever had," said Dave Glew, senior vice president and general manager of Atlantic Records in New York. The company had planned to press only 50,000 copies of the album initially, Glew said. "It was released Dec. 5 and by Christmas we had orders in excess of a million. We had to go outside our traditional pressing plant because they couldn't handle the load."

"Briefcase Full of Blues" was recorded live last summer when the Blues Brothers opened for Steve Martin during his nine-day sold-out stand at the Universal amphitheater in Los Angeles. The fact that the album currently is outselling Martin's recording of the same date points up the shrewdness with which the project was conceived, produced, packaged and promoted.

Having parlayed their "Saturday Night" recognition into a three-album recording contract, Belushi and Aykroyd lined up one of the best backup bands that Atlantic's money could buy, including Cropper on lead guitar, "Donald Duck" Dunn on bass, Tom Scott on saxophone and Matt "Guitar" Murphy and Steve Jordan on drums.

"These are all six-figure guys who did it for a good time, for some laughs," said the Blues Brothers spokesman. The band members were paid their usual fee for playing, but will not share officially in the record royalties, which already amount to at least $1 million for Belushi and Aykroyd.

"I think the theatrical part of what John and Danny are doing is what put it over initially," said Cropper, "but the reason it's hitting so big right now is because there's a good band behind it. It's not a rip-off or a mockery of the music; it's a very serious approach."

"I have mixed feelings about the record," said Vin Scelsa, a deejay at WNEW-FM in New York, one of the first radio stations in the country to air the album. "I appreciate it for what it is, a put-on, a put-on with real fine musicians behind it so it comes off sounding good. But on the other hand the purist in me is a little annoyed by it.

"In the '60s, the Beatles and the Stones did old blues material but in such a way that you could take them seriously," Scelsa said. "But I can't take the Blues Brothers seriously. If Jake and Elwood really were Jake and Elwood, they'd be just like any number of guys from Chicago who are hungry for gigs and a recording contract. It ain't the Butterfield Blues Bank. It's a joke, a very funny joke, but still a joke."

But if "Briefcase Full of Blues" is a joke, no one connected with Belushi and Aykroyd is willing to admit it, perhaps thinking that the best way to perpetuate a put-on is to deny that it is one.

"They (Belushi and Aykroyd) don't believe it is at all," said "Saturday Night Live" producer Lorne Michaels. "We set the tone of it on the show as somewhat of a put-on, but I think they were deadly serious about it all along. I think it's always been a major desire of theirs to do this kind of music." Asked if he thought the Blues Brothers were a joke, Michaels replied, "I'm the wrong guy to ask on that."

"No, No, No, No," said Barry Secunda, manager of a number of "Saturday Night Live" writers and a friend of Belushi and Aykroyd from "before the show." "Belushi's not kidding at all. Like all the children of the '60s he's always wanted to be a rock 'n' roll star.

"Lorne Michaels initially didn't take the thing seriously and neither did John's manager," Secunda said. "They kept thinking it was a phase that would pass, but John and Danny pressed on. If there's a joke on the album it would be the fact that Dan's harmonica is in a briefcase handcuffed to his wrist, but that's all. I don't think you can assume that if Belushi got up there and belched it would sell too many copies.It has to have musical merit."

Although Atlantic concedes that Belushi and Aykroyd are not exactly Sonny Terry and Brownie McGhee, "we felt we had a musically strong record," Glew said. "In the beginning we were concerned that the retailers and the consumers would think it was a comedy album. We wanted to be sure it wasn't merchandised in the stores in the comedy bins."

And so, the week before the album was released, Atlantic executives split up into teams of two and called on their key retail accounts across the country, dressed Blues Brothers style -- shiny black suits, pork pie hats, dark glasses, skinny ties, even briefcases and handcuffs -- and carrying with them videocassettes of Jake and Elwood and the boys in action.

Then, with just a few weeks remaining in the Christmas shopping season, the company launched a "major" promotional campaign. "We funneled tens of thousands of dollars into advertising -- top 40 radio, FM radio, print and TV," Glew said.

The effectiveness of the musical and promotional mix can be seen in the resulting AM radio play, Glew pointed out. The Blues Brothers cover of the old Sam and Dave hit, "Soul Man" was released as a single and now is in the top 20. Comedy albums are not known for producing hit singles, though Steve Martin's "King Tut" was in the top 10 last year.

Atlantic is planning to release a second single from the album. If that, too, becomes a hit, Glew predicts "Briefcase Full of Blues" will sell "three to four million copies fairly easily." The second Blues Brothers album will be the soundtrack to the promised Blues Brothers movie, Glew said.

"It will be a road-picture musical like no one's ever seen before," said Sean Daniel, vice president of production at Universal Studios. "Dan Aykroyd is working on the script right now and we expect to start filming this summer on location in and around Chicago, the birthplace of the Blues Brothers." The movie will be the first for Belushi and Aykroyd under their individual multiple-picture contracts with the studio, Daniels said. "Animal House" starring Belushi, was Universal's largest grossing movie in 1978.

The two comedians appear to be playing the roles of Jake and Elwood to the hilt. At the pair's insistence, the names Belushi and Aykroyd do not appear on the album cover (although they are on the liner notes) -- an omission that Atlantic Records agreed to reluctantly, according to Glew. In addition, the two reportedly are not accepting interviews unless they are done in character.

Which is exactly how they appear in the current issue of Rolling Stone. Unabashedly smitten with the "Saturday Night Live" troupe, the magazine's Blues Brothers cover story is accompanied by an album review which noted "rumors that John Belushi and Dan Aykroyd were intimately involved in this project."

In a somewhat related development, fellow "Saturday Night Live" member and Rolling Stone cover subject Gilda Radner reportedly is readying musical material for a Warner Bros. album.