CHERRY BLOSSOM time is traditionally preceded by another Washington spring tradition -- Hexagon time. "Thirty Proof," the satirical troupe's 30th anniversary benefit show, gives some more good-natured goosing to Washington's famous and not-so- famous. Proceeds from this year's outing, earmarked for Columbia Lighthouse for the Blind, are expected to put Hexagon over the $1 million mark in monies donated to local charities.

Hexagon's 60-plus volunteer performers and musicians are professionals from all walks -- it looks at times like Yuppies on Parade.

The best of the 30 original songs and skits take sly swipes at the city's more obvious targets, most often the Prez (remarkably impersonated by Don Williams) and the Metro system. Hexagon also tackles John Riggins in a sassy skit, bids a fond farewell to 14th Street's sleaze and flies high with a clever bit about an airplane's close brush with the USA Today building.

Comedian Joan Cushing, aka Mrs. Foggybottom, is back and better than ever, and the proceedings are punctuated by several "Hexagon Update" news spoofs, featuring a rotating roster of celebrity announcers, including WRC-TV's Arch Campbell and Bob Ryan, WJLA-TV's Paul Berry and WMAL-AM's Chris Core, Ed Meyer and Tom Gauger.

The troupe has brought back a handful of "greatest hits" too: a funny skit about how Pierre L'Enfant really designed the capital city, a mock oratorio about rush-hour traumas, and the bump-and-grind Hangar Club number, which gives the Hexagon gentlemen equal time with the annual ladies' kickline.

The whole effort is infused with fun, like a collegiate vaudeville revue. Sure, there are plenty of clunkers and groaners, but that's half the laughs. And, as they say, it's for a good cause. THIRTY PROOF: THE 1985 HEXAGON CLUB REVUE -- At Trinity Theater (36th and O streets NW) through March 30. Call 342-9600.