It refers to someone where they have taking a piss through there underware and left it for 1-2 weeks as it was left moist, mould starts to grow in the crotch regeion giving the effect of mouldy underware

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.