Perhaps there was a bumch of guys standing around one day, and naturally they were trading sex stories for shits and giggles. Then one of the men came to the topic of oral. At first he used the literal term of a "suck" job, which caused immediate confusion among the circle of friends. They mistakenly took him for saying that the oral sucked, in a bad way. They scrapped that idea and decided if they were going to blow their loads, they might as well call it a "blow" job. How did they get the term "load"? Well that is a whole other story all together.
LOL