Black beans are also called navy beans or turtle beans. Their skins contain an intense dark-purple pigment, and the intention here is to keep this colouration, as far as is possible. This recipe makes rather more than a pint - enough to cover 6-8 medium-sized baked potatoes - and it takes about ten minutes in the kitchen, and about an hour in total (excluding soaking time).
Soak half a bowl of black beans overnight or for a few hours. Drain and rinse, and then boil in plenty of water until tender (about 20-30 minutes).
Chop a large onion and fry in olive oil (and maybe some butter) in a medium pan until golden. Add 2-3 cloves of chopped garlic. Add the beans to the onions, along with any remaining cooking water (you can drain them first, but I prefer to keep the water, because the inky blackness is a feature). Add extra water as required. Add cumin, chili, salt and pepper to taste (the chili will get less strident with cooking, so check again later). Bring back to the boil, and simmer until the flavours have blended (about half an hour). The result should have a consistency something like Baked Beans, but less watery; a rich, spicy flavour; and an inky-black colour.
Dilute and reheat to make a soup; or serve with cheddar as a topping for baked potatoes (recommended), or maybe with rice, or even with pasta.
It's definitely best the next day. This stuff seems to be highly attractive to fungal spores; it won't last till the third day!