Thank you again Ruth for a rational and researched comment.

As to some others, as I have said, offence may be taken based on ignorance, and that should not be pandered to - but it may well be wise to explain the correct position.

That position would (on the evidence considered here so far) seem to be that there has been a pre-minstrelsy custom of guised dancing, and that the colour chosen was likely in some cases to have been a matter of convenience but that in some cases it was black.

On that basis it still seems perfectly proper to state (as I did) that the genesis of blacking up was not racist.