That was a reeeaaally long soak time! I would expect them to be pretty soft by that point. Black beans usually cook up pretty quickly. Did you cook them with an acid (like lemon juice)? That could have affected the cook time. My first guess though, is that you have old beans. I hate to say it, but if you didn't add lemon juice to the cook water, I'd ditch the beans you have and buy some new ones from a store that has quick bean turnover.
Black beans work well in a crock pot. Soak the beans (a few hours is fine - anything longer than overnight is overkill). Rinse them and pick out any stones (usually there aren't any, but it is a good idea to check). Then add 3 cups of water per cup of dried beans, a whole scrubbed (but not chopped or peeled) onion, and some kombu seaweed (if you have it) to the crock. Cook on low for 7-8 hours. If you are making beans ahead, you can freeze them at this point, or use them in a recipe. The only bean I've heard you should not cook this way are kidney beans, because they need to be cooked very hot for a certain amount of time to be safe, and the slow cooker will not get hot enough.
But if you have a pressure cooker, that is the absolute best way to cook beans. Here's how I do it: Soak and rinse the beans. Put the rinsed beans and water in the pressure cooker. For every 1 cup of beans that you put in the pot, add 3 cups of water. Add 1 scrubbed onion and 1 piece of kombu seaweed (if you have it). Bring up to pressure and cook for 15 minutes, then let the pressure drop naturally. The beans will be perfect!
I do add salt when I cook my beans, and never have a problem with it making them hard, but some people swear it makes a difference.