well THAT went well...

Well, in the vast majority of cases, once you stop giving in, nothing happens. The person who'd doing the blackmailing does it because they have leverage, and telling everyone deprives them of that leverage. She could tell everyone, but then she loses the secret and any chance of a friendship she has. And often, people aren't willing to make such a trade-off.

And even assuming she tells everyone, then it's still your word against hers. You can just claim it's her trying to make you look silly, and that you don't know what she's talking about. Unless she can somehow get hard proof of you being bisexual, she hasn't got a leg to stand on. Just stay calm and she'll look like a girl who's trying to make herself look interesting.
(and, on the bright side, you might use it to gauge reactions. Maybe they are way more accepting than their tough talk indicates)

Sure, it might be technically lying, but if you aren't ready to come out to the others, I'd say it's permitted. Coming out is not something you should do before you're ready, after all.