As far as I know it doesn't "disappear" unless you actually removed them, if so, it would be the first time I hear about it.
Thing is, log off the game, and then back into the game.
The first map you are in, the block list and friend list is in it's "working" state.
Only then you are able to add/remove someone to the block list, scan through and see if they're there then, if they're not they should be unblocked already.
Logging off and then back on again is always the best way to ensure changes.
