That it went completely dark makes me think it is a fuse. There are other fuses hiding around on our car...IDK if hvac control has one built into the back of it, but that's possible...and then there are fuses in the e-box (under hood) and behind fuse panel.
Check Bentley or spaghettidecoder.org and see if you can find a blown fuse and/or maybe a burned up wire/connection.
If you have any aftermarket electronics installed at all, I'd begin by looking at them.
First time I recall reading about the control unit going dark, so hope you figure it out soon.
I'm so depressed. No one wants to come see me in NYC: Sat, Oct. 8, 6 p.m. at Greenwich Village Comedy Club.
Mangofy your E46!