Why do I need to wait 24 hours to block a user again after unblocking?
At the time you are unblocking someone, we are already warning you in the popup that you'll have to wait until you can block this person again.

We implemented this to prevent users from abusing the Unblock/Block feature, making other users think that people can bypass the Block feature. This could be used for harassment, and we don't see any reason why would someone want to unlock a user for less than 24 hours.