I think the blazer with jeans thing needs considerable understanding of suit jackets, blazers, sport coats, etc. What makes them tick. What makes them formal, casual. Shoulder structure, lapels, length, texture, pockets, details, etc. When you look at successful casual blazer fits, you should be able to point out what about the blazer makes it work.
I think that understanding the MC-ish aspect of jackets helps considerably.