I scraped my basement walls and repaired mortar joints. I painted walls with 2 coats of Drylock. I noticed what appears to be black mold forming in several different spots on the walls. The basement isn't wet and I have a dehumidifier running all the time.

Remove with a bleach solution as per the CDC recommendations:
CDC - Mold - General Information: Facts about Stachybotrys chartarum and Other Molds

Mold grows in moist areas, so you might check
a) your settings on the dehumidifier, and
b) make sure that it is working correctly.