The moral of Kin Platt's slightly wacky thriller, "The Blue Man," seems to be that any red-blooded American teen-ager is bound to be more than a match for any blue-bodied whatsit that might choose to drop in from the Milky Way or from that house down the street (the one where the shades are always drawn). Another moral, this one for authors, is that when you trot out a villain with a lovely cerulean complexion, your explanation had better be good. Unfortunately, after Mr. Platt has introduced his blue bully-boy and allowed him to strike down the favorite uncle of our hero, Steve Forrester, there isn't much left except The Chase. Steve plunges off in pursuit of the fellow (or things) and pretty soon there isn't a lawman from Maine to New York who needs to be convinced that it's true what they say about the younger generation...