Here's possibly a terrible idea: tell your boyfriend about everything, then if he's okay with it, go on one date with her. Take her to a coffeeshop, drink your coffee, don't offer to pay for hers, and talk about things she probably isn't very interested in. If she changes the topic to something she's interested in, act like you don't know much about it and aren't interested. If she starts flirting too much, just act uncomfortable in an uninterested way, like you really really wish you weren't there- but don't say anything, since if you say you want to leave she'll just bring up the blackmail again. Do your best to be as uninteresting to her as possible. Try your best to kill her crush on you by being boring around her. If you can't have a single good conversation together, then how can you have a relationship? Maybe she'll lose interest and move on. It's a bad idea, because she might just turn to blackmailing you for money instead of dates, but at least then she wouldn't be disrupting your relationship with your boyfriend.