Just because TV series heroes appear to be getting older, that doesn't mean that television entertainment is becoming more mature. It's just, by and large, mining different veins of childishness. The phenomenon might be called the "Cocooning" of the airwaves.

Still, "Blacke's Magic," the new NBC series that premieres as a two-hour movie tomorrow night at 9 on Channel 4, is a little less appallingly puerile than most.

Although NBC is No. 1 in prime-time ratings and thus might be considered to have new latitude for experimentation, the great creative minds that do the programming have instead come up with this -- a bald imitation of the CBS success "Murder, She Wrote," concocted by some of the same creative team, including Peter S. Fischer, executive producer of both shows.

In place of a little old lady mystery writer who solves crimes, the auteurs of "Blacke's Magic" offer a retired magician who solves crimes. As if Hal Linden weren't old enough (he plays Alex Blacke), he is given an even older sidekick -- TV perennial Harry Morgan, veteran regular of 10 previous episodic series, as Blacke's lovable, codgerly con-man father. Harry has "comic relief" written all over him.

On tomorrow's premiere (the series begins its regular run Wednesday night at 9), it takes about 45 minutes to get Blacke into the sleuthing business and set up some hint of a plot. An old crony of Blacke's, the Great Gasparini (Cesare Danova), gasparinis his last while performing an underwater escape at a magicians' convention. The mystery: How did he die with a bullet through the heart when he spent the last five hours of his life submerged in a coffin under 10 feet of water and in full view of the human race?

As with the old "Perry Mason" series, a great flock of suspects is fluttered across the screen to complicate the situation, among them David Huddleston (Santa Claus in "Santa Claus: The Movie," but looking much jollier without that mangy beard) as the head of the American Magic Society; Gary Frank and Joseph Cali as a pair of shifty trick-plagiarizers; Maud Adams as a tough local TV news reporter, whom everyone refers to as "Miss George" (first name, Teresa); and others.

Kathleen Beller, in ultrashort ultraskirts, plays the dead magician's daughter; Mark Shera, once sappy patsy to Barnaby Jones, plays Blacke's son-in-law-to-be; and Claudia Christian plays his daughter Laurie. Funny magician Carl Ballantine has a cameo role that seems to owe a little something to Christopher Lloyd's mad performance in "Back to the Future."

Linden, who was always the unfunniest thing about his series "Barney Miller," brings a nice seedy charm to the part of Blacke. Morgan is up to the same tricks he's been doing through three dogged decades of TV sitcom work.

Fischer's dialogue rummages through the TV crime-show vocabulary: "Look at me when I talk to you!" "Get your hands off me!" "Let me give it to you straight, honey." And the ever-popular, "You know, Phil, you're disgusting." Director John Llewellyn Moxey shined things up nicely, though even he seems bored with such digressions as the gratuitous opening-scene stunt, one that has nothing to do with the mystery to come. It really is a mystery, though, clever enough to be at least a wee bit surprising when all is revealed.

"You know, Alex, there's just one thing I don't understand," Beller says in the course of the denouement. Ah yes, there's always that one thing. Blacke, for his part, pauses during the investigation for a dabble in philosophy. "It doesn't always come out even," he observes, "but you can't let it turn you sour." That sounds like advice to a lifelong viewer of network television programs.