@Spain10 that's the thing, you don't need to trust the girl, you need to trust your boyfriend. In a relationship that's truly built on trust and security you wouldn't need to ask your boyfriend to block a girl he used to be interested in. What exactly does that accomplish, anyway? He can always unblock her, create a different profile, or any number of other ways to contact her. So ultimately you'll constantly have to be checking your boyfriend's phone to ensure he's not talking to her.
Things like asking our partners to block/unfollow people or checking their phones gives us an illusion of control, like we can stop them from leaving or betraying us through all these little invasive actions. But ultimately it is an illusion. If our partner really wants to cheat on us we can't stop that from happening. These little "ressurances" like showing us their phone don't actually do anything to address the root of our insecurities. Your efforts will be much better spent building trust and security with your partner than trying to monitor their phone.