Mayim Bialik is feeling a little out of place. While other L.A. 17-year-olds are hitting the beach, the ivory-pale star of NBC's Blossom is clad in black ('it's all I wear''), sipping coffee at a Hollywood cafe, and confessing that she would pass up mall-going any day for ''reading and thinking. I'd rather talk about Somalia.'' To remind the world that she's more than a spunky sitcom teen, Bialik will play a wild party girl in an episode of Lifetime's women-with-secrets series, The Hidden Room (July 17, 10-10:30 p.m.). ''I didn't want anything with a laugh track,'' says Bialik of the move. ''Drama is what I did before Blossom''-most memorably as a young version of Bette Midler's character in Beaches. Nevertheless, it is the popularity of Blossom, the top-rated show among female teens, that has defined her recent life. ''I still find it strange that people want to know what car I drive, or what my favorite food is, or who I'm having sex with, or what my SAT scores are!'' (A Saab, macaroni and cheese, no one these days, and she won't tell-though she gained admission to Harvard, which she has deferred for a year.) Bialik also knocks reports that she is furious about the teen-idol status of Blossom costar-cum-pop singer Joey Lawrence. ''Come on, already,'' she pleads. ''I'm sorry if this bores people, but we do get along.'' Her stereo, however, is more likely to be playing her faves Elvis Costello or 10,000 Maniacs. Her other passion: jewelry. The charms in her nine pierced earholes and on the chains around her neck include a mini silver razorblade and a Star of David. ''I guess that's because I'm a sharp Jew,'' she jokes. In the April issue of Mad magazine, however, Bialik was portrayed as dull and whining in a parody piece titled Blah-sum. ''It was full of cheap shots,'' she says, ''and I loved it. My first thought was 'Oh God! Why me?' But someone later told me, you know you've arrived when Mad parodies you. So I guess I've arrived. I even sent them a thank-you letter.''