Someone who is anything from ignorant, rude, annoying, a dink, a dummy, or an asshat. Basically a substitute for any insult imaginable.

A Bloomer is someone with a bright outlook on life who is optimistic in nature. Bloomer's frolic in every opportunity to be happy and try to make the most out of life. The opposite of a Bloomer is a Doomer. The term Bloomer originated specifically as a counter to the Doomer on sites especially like 4Chan.