When is it ok to use black paint to cover unwanted pavement markings?
The answer is never. This isn't a good idea because the black paint can fade leaving the old marking visible. Sometimes you can see right thru black paint if light hits it at the right angle.
Paragraph 04 of Section 6F.77 "Pavement Markings" in the CA MUTCD says the following. Notice the use of the word "shall", which indicates a standard that must be followed.
04 For long-term stationary operations, pavement markings in the temporary traveled way that are no longer applicable shall be removed or obliterated as soon as practical. Pavement marking obliteration shall remove the non-applicable pavement marking material, and the obliteration method shall minimize pavement scarring. Painting over existing pavement markings with black paint or spraying with asphalt shall not be accepted as a substitute for removal or obliteration.