Why’s it called a blow job when you’re mostly sucking it?

Because the term started life as "A below job - i.e. below the belt - and mutated to blow job because whispered quickly, in the backseat of a car, that’s what it sounds like.

I stopped smoking 10 years ago. Once in a blue moon I’ll subconsciouly reach for the non-exsistent pack on the table, catch myself and laugh, whilst thinking about the money I’ve saved.