Sourdough is stable because it's a mono-culture. The wild yeast in the starter is more robust, and able to withstand higher acidity than is store-bought yeast from a grocery store. Because of this fact, they stay active when the acids they give off after eating the sugars available in the flour and any added sugar. Eventually, the sugars are mostly gone, leaving live yeast, protein, fiber, and acid. The acid is what gives sour dough its sour flavor.
If you add baking soda, you neutralize the acid, getting rid of the sour flavor. This allows other opportunistic organisms to grow in the more neutral-pH environment, such as molds that commonly float around in the air. Mold grows quickly and could very well be what is creating the black spots. Plus, adding baking powder to the starter, or dough, negates the desired sour flavor of sourdough breads and pastries. Don't add baking soda. It does not leaven the bread. It does not improve the flavor, and will ruin the texture. Also, the bread dough won't rise properly.
Seeeeeeya; Chief Longwind of the North
“No amount of success outside the home can compensate for failure within the home…
Check out my blog for the friendliest cooking instruction on the net. Go ahead. You know you want to.