After fucking your chick (or dude) in the ass, you pull out your dick and slap her (or him) in the face thus giving them a shitty mushroom stamp hence the chocolate mushroom stamp.

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.