That a blowjob involved the act of physically blowing air on the penis. When I found out it actually involved sucking, I was like, "Oooh...yeah that sounds much more pleasurable."
I was so confused, I couldn't imagine why people would enjoy that more than a "suckjob" or "headjob". Turns out people just say whatever they want and it can mean anything.
In my language the expression is "jemandem einen blasen" (to blow one to someone) and I remember reading a long time ago about a story where a teenage girl (?) actually injured her boyfriend when she blew into the penis. Seems not to be a very good idea.
In our language it's to 'pull' someone ot better "jemandem einen ziehen" as we are civilised and have grammatical cases such as dative. So I hope nobody got it confused and ripped of someone's penis.