I always use natural colors in clear water. That being said, black worms can imitate leeches very well so if the water you are fishing has a population of leeches - then by all means...use them. I generally reserve black for stained or dirty water.
Depends on the fish. I have been fishing for a while an know that sometimes there are exceptions to some of the advise that has been given. Sometimes bass are picky eaters and sometimes not,. With clear water fish they rely more on their sight to feed. However, in our lakes down here in Texas we fish green pumpkin, watermelon, or any type of dark green plastics all year round even when the lake visibility is clearer.