There is no such word as "bluishly". If the coat is a bluish green it means the colour is green with a hint of blue.

It is an adjective. -ish is a rather modern suffix meaning somewhat. Bluish- somewhat blue. You can use -ish on the end of most adjectives when you're being informal. Can also be used to show rough time - the show will start at 7ish. (About 7)