Unique blips are in essence just new entries in the blip legend when you are on the pause screen map. Reducing these are pretty simple:
Instead of having one blip called “MRPD”, another one called “Rockford Hills Police Station” and one called “Sandy Shores Sheriffs Office” etc. you would merge them together and call them “Police Station”. Or maybe divide them into two groups so you get “Police Station” and “Sheriff’s Office” etc. Whatever fits your needs.
NOTE: they must also have the same sprite, name etc. for this to work. They will end up on the same line in the blip legend if you did it right.
And when it comes to the already mentioned native SetBlipCategory: It can be used to group other things than properties, I’ve found it useful for grouping police officer blips like so:
This would organize the police officers in a category like this: