I eat black beans several times a week, and they never seem to stay black. Times they turn to a dark brown, but definitely not black. I start with dry beans and usually soak them during the day when i'm at work, so when I come home they are ready, but sometimes I soak them the night before, then drain them and put them into a ziplock in my fridge for the day. I notice when I drain the water the water has a lot of black in it and the beans have a deep purple color, not really black anymore.
When I get home from work, the soaked beans go into the pressure cooker (using the same water if I soaked them during the day, fresh water if they were soaked the previous night and in a ziplock). Over time i've played with the amount of time and water in the pressure cooker, but got it down pretty good now, so that when they are done, there is still about 1 inch of water over them. After the pressure cooker, I take 3-4 cloves of garlic diced up, brown the garlic in olive oil... and just went the garlic is brown and toasty, dump the beans and water into the skillet and throw in some salt. I then let it sit on a med-high heat for 30-45 minutes until much of the water has reduced and it's more of a thick sauce, stirring occasionaly. They taste great, but they just aren't black. I notice that they are not black out of the pressure cooker, but I don't think i'm cooking them too long. They are still firm and not falling apart when I take them out... maybe pressure cookers just do something to them? Usually I use the bulk black beans that cost just under $1 per pound, but i've also tried the more expensive ones that cost over $2 per pound... the only difference i've ever noticed is the more expensive ones are usually cleaner with less rocks.