How many of you actually use the blacklist feature in gmod ?

The function is quite useful, but still there are drawbacks.

Assuming OP is referring to Blacklist in the Legacy Server Browser, I don't know, but it could be useful for minging to blacklist servers you're banned from.
The Props blacklist is often used to help prevent prop flying, kill, etc / prop spam server crashing.

I use it for servers im banned from its really useful for that so you don't try to join any servers your banned from

blacklist overrated and useless, use the whitelist!

I use it for DarkRP for when you NEED to prevent people from propspamming.