Am I at fault?

I was backing out of my driveway and hit my roommates car who sticking out into our driveway more than 5 feet.  The spot he parked in was a 10 foot spot with a residential mailbox (obviously not a parking spot).  His car is 15.9 feet long plus license plate. We have told him multiple times not to park there. This time I was backing up and lightly scraped his bumper. I was just going to fix the scratches but he wont drop the subject. Would I be legally responsible to fix the scratch? I live in Southern California.

Yes, you are at fault.

Backing up requires you to be 100% aware of what's behind you.

If it was a child behind your car, you'd be just as negligent, and just as responsible, even though the child didn't belong there.

What makes it even worse for you is that you already knew that your roommate had parked that way before.

I can't imagine how you didn't see that part of his car unless you flat just didn't look. And that's negligence.

Since blocking a driveway is a violation (but not necessarily negligence) you had plenty of opportunity in the past to get your roommate cited for parking that way but you accepted his behavior with prior knowledge and that works against you.