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 (http://www.cdc.gov/mold/stachy.htm#Q11)

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