Cook's Illustrated claims baking soda helps black beans retain their color instead of fading to purple. One of the linked sites dismisses this as an old wives' tale, but Cook's Illustrated doesn't rely on old wives' tales. They test everything. The amount they recommend, 1/8 teaspoon, appears to be a small enough amount, according to the linked sites, that it doesn't significantly affect the nutritional content of the beans.
They also tested the question of adding salt early or late and found that adding it early results in better-seasoned beans and doesn't keep them from softening or significantly affect the cooking time. So they say go ahead and add salt early.