Abbrevition for blood. but its a way of endearing somebody or adressing 'em. It definitely don't mean "boo" ,whoever the person was that wrote that.

A greeting to one of your homies

B
The word is a derivative of Brother, was, through usage, shortened to Bro, and finally condensed to just B.

Asian Fail.

The second letter in the alphabet.

A colloquialism one would use when referring to one of their homeboys/girls, or, more generally, anyone. Synonymous with other nicknames like "ese" and "playa."

b is short for bag, the verb. bag is used to mean that someone is going to hook up with someone else. so when someone is trying to bag someone else, as in they are flirting or starting to make moves, one would make his/her right hand into a b (thumb straight up, fingers curled behind curled pointer finger) behind the person who is trying to bag. in addition, someone could say "b" or type it on facebook, text it, etc.

It's a shortened version of Babe or Bitch, depending on who you're talking to and how you mean it.