Well GrannyTwice, I have only just come into this thread and I notice that you want to know whether there is a difference between a black mark on the wall and giving someone a black mark. Also, perhaps, the difference between a black mark of any kind and racistly referring to someone as a black person.
1) A black mark on a wall is a mark or stain which is of the colour black, as defined by 0 red light, 0 green,0 blue. No mention of people of any race or shade whatsoever.
2) A black mark given to someone is literally or symbolically a mark made on a sheet of paper in black ink (as defined by 0 red light, 0 green and 0 blue) No mention of people of any race or shade whatsoever.
3) Referring to a person by using their ethnic origin or skin colour as "black" is a hangover from the time when slaves of the colour black (or white) were considered inferior to those who were not enslaved. Specific mention of people of black colour or African ethnic origin.