Whenever I try to add something to the blacklist, the page just refresh and a green banner says "Email has already been taken"

What gives?

this happens when there are somehow two accounts with the same email associated with them. see: forum #367524

sipothac already gave a really good answer and linked information about it. I'll just add that basically this happens because it needs to be 1 email=1 account. You can have multiple e621 accounts (as long as you don't use them to rule break or ban evade), but each one needs to have a different email attached to it. We used to be less strict about this, so some people have more than one e621 account that were all using the same email address. Or an old one they forgot about, etc. So then when it tries to save your settings/blacklist, it finds that email is still listed for more than one account and then it can't save like that.

So to fix it, you can either A) change the email to something that is not being used for another e621 account for themselves, or B), use the method linked in the forum above to find the old/other e621 account that was still using that email, and consider whether you still even want or need both/all anymore, and delete the other account, etc. Either strategy will work to fix it, so you just have to decide which solution works best for you.

But either way, once that detail is changed, then it should let you save your settings and blacklist normally again and without any further problems.