How long should I soak black beans before I cook them?
Overnight, if you can -- but if you don't have that much time, you'll just need to simmer them a little longer. Or you can do the quick soak: bring the beans up to a boil, then let them sit for 30 minutes to an hour to shave off some simmer time.
Overnight. Soaking in salted water allows them to rehydrate (which is important for reduced cooking time) but also releases some of the indigestible sugars (which is why you should always drain and rinse the beans and start with fresh water for cooking). Of course, long soaking is not mandatory and beans can be boiled and simmered for 1/2 then rinsed and cooked or can be slow cooked from dry, but you will have the most control over the texture if they are plump before you cook 'em. (Oh, also, for best texture and to control saltiness don't salt your cooking water, wait until the beans are just about cooked and salt to taste at the end.)
I agree with the other posters--there's more than one way to cook a bean...
Before I had a pressure cooker I would most often use the Russ Parson's method, discussed here: http://forums.egullet.org/index.php?/topic/99104-no-soak-beans-in-the-oven-in-90-minutes/ which is basically to bring dried beans to a boil in salted water, and place the covered container in a 250 F oven for 75-90 minutes, or until the beans are done.
Now I brine the beans overnight, as Slow Cooked Pittsburgh suggests and cook them up quickly in the pressure cooker.