Blanket is just consistent visual spacing between objects, most prominently between sliderbody/streams and anything else.
A blanket is considered "bad" when the spacing is not consistent.
Since blanket is more of the aesthetic side of mapping, those so-called "bad blankets" don't have any severe effect on the map unless your map was designed around aesthetic where inconsistent blankets will irritate people with OCD. Also, try to map blankets where possible when aesthetic isn't your strong point can make your map looks sloppy instead.
Overlap isn't bad if you know what you're doing. Overdoing it may interfere playability, and can also ruin your map aesthetic.
TL;DR everything can be a two-blade knife. Master how to use them.
Personally I don't care much about overlaps anyways. Map with your own way is the best I can give you.