It's the first 2 letters of the GFY'er alphabet.
Why it wasn't named "suck job" i'll never know.
Origin of the term "Blowjob"
******** (n) an act of fellatio.
This term, now widespread in English-speaking countries, spread from the USA in the 1960s. A puzzling misnomer to many, to blow in this context is probably a euphemism for ejaculate, a usage occasionally recorded in the 1950s. This may itself be influenced by the there she blows of whaling cliche. An alternative and equally plausible derivation of ******** is from the black jazz musicians hip talk expression blow meaning play (an instrument). This term probably caught on in Britain and Australia simply because there was no well-known alternative in existence.(1)
Source(s):
http://www.askthecouch.com/slang.asp