Easy one dude.
It's called "blow" because:
1. Sluts will do it in exchange for blow (coke).
2. When she is doing it, if she really doesn't like you all that much she may be wishing she could blow you away instead.
"Job" comes from:
1. It really is a job to them. See the above.
If the girl doesn't do drugs, she is still doing it in exchange for something i.e.: you providing shelter, food, etc. Or maybe they want jewelry.
Finally, in the best case,
2. the woman really loves you and is trying to please you. This makes it a "labor of love". Job fits here too. It should be noted that this is the best case scenario and is extrememly rare. In this instance, such a giving attitude should be repaid with frequent returning of the "favor".
They love that and it makes them feel loved in return.
Hope this helps!