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Judge DredI guess it would depend on the mark.
For example: The N300 Mayo cards have a black back and a black border that surrounds the front edge of the card (similar to the 71 Topps BB cards). If a BLACK marker was used on it then PSA or SGC would probably reject it for color being added.
If it was a black mark on a white (or other color than black) back ground then you probably have a pretty good chance of it being graded by PSA with the MK (mark) qualifier. SGC would probably also grade the card but they don't have qualifiers which means that they may penalize it a bit more for this type of defect.