A patch of blue appears on the snow, and then another, and another, and another and another and another, and we hear at last that familiar and jubilant and altogether meaningless cry, "la la la la la la, la la la la la." Who could it be but the Smurfs, starring in their own inimitable and inevitable Christmas special?

One may have been able to get along very nicely without "Andy Williams' Early New England Christmas" and even without "Perry Como's Christmas in Paris," but "The Smurfs Christmas Special," tonight at 8 on Channel 4, seems somehow more crucial, and it brings the wee, tiny, and otherwise infinitesimal stars of NBC's Saturday morning lineup to prime time for a half-hour of merriment, fairyment, and just plain smurfing it up.

What is it about the Smurfs? They toil not, neither do they spin. Well, actually, sometimes they do toil -- as when preparing the annual Great Pudding for Christmas -- and they do appear to have a few spins in 'em, but this above all may be said of the Smurfs: They mean no harm. And even if they did, they would be incapable of committing any because they are too small. Oh, there's a lot we wicked old human beings could learn from them, I'll tell you.

But all is not well in the woods as the Smurfs prepare for the holiday. Two lost children have fallen out of a sleigh and are stumbling about, the evil wizard Gargamel has learned how to reduce the Smurf village to rubble, and a mysterious bearded stranger who looks a little like Bob Fosse has been tossed in to complicate an already overcomplicated and generally insmurfable plot.

"Oh, Smurf is us," cries one of the Smurfs when it looks like all is lost. But the Smurfs save the day -- days must be saved in cartoons -- with a few idiotic choruses of "Goodness Makes the Badness Go Away" (and a snatch or two of Tchaikovsky's Sixth in the background) and an uneasy calm returns to Smurfadelphia. Smurfs indeed. Great Pudding indeed. Indeed indeed -- or, to put it more smurfishly, yes, indeedy-doo. How did that song go again? Oh yeah, la la la la la la . . . 'Powerhouse' ---

Teen-agers are told they ought to maintain "an 'I can' state of mind" in "Powerhouse," a new public-TV series aimed at them that premieres today with a special one-hour episode at 5 on Channel 26. As with similarly well-meant public-TV projects, one may wonder if those who would most benefit from the message might also be those least inclined to watch, but at least this program puts the message over less sappily than is usually the case.

Sandra Bowie plays Brenda Gaines, a young woman who wants to turn her late father's now-dilapidated gymnasium into a youth center in the show's opening installment, which was filmed in and around Washington's non-monumental neighborhoods. Young toughs are running numbers out of the old building, and in a series of far-fetched plot turns, Brenda and friends drive them out of the neighborhood and even lead the police to the numbers operation's Mister Big.

It is a sign of the times that Mister Big turns out to be not a mister at all.

Since kids are accustomed to TV shows being interrupted, a number of commercial breaks are thrown in, but the commercials are really bite-size and palatable pieces of advice, like, "Participation is the name of the game," "In an emergency, call for help," "Think before you act" and, oh yes, "Wear shoes that fit right." A short animated spoof of a TV game show is called "Celebrity Organ" but not to worry: the organ in question turns out to be Fonzie's lungs.

Bowie brings conviction and heart to a role that could have been just another Miss Goody Two Shoes, and Michael Mack makes a solid, easy- going contribution as Kevin. The cast is really the program's strongest suit. It's what keeps the script from going squishy and the show from being merely illustrated preachiness. Thanks to the kids who live there, "Powerhouse" packs just the punch it intended.