It happened to me also. I had used my camera earlier yesterday to take photos of myself (the camera on the front) and I left it set to the front camera as the default. I was using on of the millions of facebook apps on my EVO and I clicked on something that said, "take a picture" it tried to go to the camera but the screen just went black. I could go "home" and use the phone, but if I tried to turn the camera back on the screen would go black.
I wanted to try trial and error to fix it but I went out to the club last night and saw a really sexxy chick that I wanted to snap a photo of (this is the best "dark nightclub" phone I have EVER seen or used) so I pulled the battery and turned the phone back on.
Everything was back to normal.
I get an app a least once or twice a day that hangs up, but the phone gives me an option to force close. But no other "black" screens.