What: Review of Blow Up, a music and fashion show. Where: Clubhouse 24. Punchline: Pay no attention to the timid man behind the curtain. It was raining Saturday night in Midtown. Not drizzling, raining. It was coming down so hard that any heads of hair not beneath an umbrella or the hood of a raincoat had that fresh-from-the-shower look. Meanwhile, on the sidewalk outside Clubhouse 24, a newly opened party space at 24th and J Street, there was some kind of sculpture/assemblage in the form of a tree. Christmas tree bulbs the size of disco balls decorated its branches, and it was on fire. Well, more accurately, it had been rigged with a large propane tank so that flames shot out of metal