Requested feature overview description.
what it says on the tin. ability to blacklist by user ids

Why would it be useful?
blacklisting by username works, but if the blacklisted user changes their username, the blacklist entry will break. it can be very difficult to figure out what the person's new username is. also it can be difficult to notice in the first place that they have changed their username. blacklisting by user id numbers would eliminate completely all the hassle related to the possibility of someone changing their username.

What part(s) of the site page(s) are affected?
blacklist