Why are chat and forum blocks seperate?
Why not just use the one IPB Blocked User List?
If I block someone on the forum, odds are I want them blocked in the chatroom, no?

Not necessarily. And the existing interface for forum blocked users clearly says "posts", so we have to make a big assumption in blocking the private chats too (you'll notice even private messages is a separate option from posts).

I thought about it, but felt it was better to have a separate option for chat. I would have preferred plugging into the existing interface (i.e. just add another option onto the existing manage ignored users screen), but it's not possible at present.