If you have PMs with someone, then they block you, I feel like you should be able to close those PMs, even without the blocking part. Also helps allot with only keeping active chats open, and not having to scroll through so many people to find someone you want to talk with. This could easily be added with either holding the profile of the person you want to close PMs with to open a small option screen, or do what most companies do and have a slide menu under their profile in the messages category. It is a simple but nice feature to have in the app.