Good question. I've given it some thought and here's my
answer:
After licking and sucking(inhaling) for some time, the performer
have to exhale & breathe out. That action is equivalent to
blowing.
Calling it a 'suck job' doesn't sound nice, an 'exhale or
breathe out job' is ridiculous.
So it was decided to call it a 'blow job'.
