I'm reluctant to join in this discussion, and have felt no need so far because Ruth, Royston and others have argued so well for my own point of view, but I feel a need to add my voice to those who find comments like this "I do not feel that black people should be offended by people browning/blacking up" to be wilfully misguided.
It is not up to us to decide if someone should be offended or not by our actions. It is up to decent people to seek to avoid causing offence - as a matter of common courtesy, (or good manners to use the traditional term). And given the current political situation dancers should be thinking even more carefully about this practice, and how it may play out to the wider world.
In 'closed' situations where there is an opportunity carefully to explain all the historical precedents (innocent and less so), black make-up might just be justifiable (as it might have been in this case), but in the street and on websites etc. dancers would do well to bear in mind that Morris is colourful, noisy and newsworthy - so punches above its weight in terms of public relations. Their choices will colour people's views on the whole folk scene.
I'm well aware that folk music tends to attract people of a more conservative disposition (note the small c), and there may indeed be people with right wing / racist views dancing morris. These will no doubt hang on to their practices for more than one reason, but others need to beware of a 'territorial knee-jerk' reaction to change.
The time line, as I see it, seems to go like this:
1) Origins of morris - possible parody of black skin = dodgy.
2) Guising to avoid recognition - innocent, but open to misinterpretation = dodgy.
3) Assumption of minstrelsy fashions - parody of black skin = dodgy.
4) Re-invention and modern popularisation - largely innocent but open to misinterpretation = dodgy.
Trying to drill down on the details misses the main picture, which is that hanging on to this 'tradition' when there are viable alternatives is not really tenable or sensible.
I know dancers enjoy shocking people (I remember my, then, very small son being freaked out by being chased by a dancer with a horse's skull on his head - the man plainly enjoyed his terror), but there are limits.
Rather than writing to a hard-pressed head teacher who made an innocent mistake, I would rather write to the powers that be in the Morris world. Forgive my ignorance but there seem to be a number of rival factions.