re: Who is at fault in this accident?

I have no idea what the law says here, but I'm inclined to blame A. Every driver should be fully aware of their surroundings at all times, full stop. It was A's responsibility to know that B was passing on his right.

Mirrors exist so that you know who is coming up behind you. Use them.

Edit: If driver A had his turn signal going indicating he was turning right, then shame on B. But since he actually wanted to turn left I'm guessing he was indicating a left turn and then suddenly swerved to the right.

Edit 2: Just reread this and realized I had it all wrong. I thought B was passing on the right, not the left. I'm thinking it's both parties that should be taken out to the woodshed now.

A for not being aware of his surroundings.
B for being so damn eager to pass that he went into opposing traffic and ignored A's blinker.