Well, you have Arucard's description above, where you use it as a vulgar challenge or to mean "I don't care about what you think, you can just blow me". It can also be used as an expression of surprise, a shortened version of "Well, blow me down!" (meaning, you are so surprised that a puff of air could knock you over.) May I ask where you are getting these phrases from? It sounds as if you are reading them from a "book of idioms/slang", instead of finding them in real English usage.

it's vulgar i suppose, like saying suck my ... im refering to word blow-job. sorry for such answer

Blow me is a British expression. You use it when something is really surprising. Blow me! The car was stolen! In american english it means what the first guy told you. lol

it means hit me, in violent way