As I understand it, Morris is a mutation of the word Moorish. I.e. Dancing in the style of the Moors who, to most people's understanding in the mid 15th century were black. I have also been given to understand that that is the origin of guising and that it was intentionally 'blackface' in the same way Al Jolson was.
Doing something for 500 years may make it traditional, but it doesn't make it right. An opinion shared by the Bard of Avon:

HORATIO
Is it a custom?
HAMLET
Ay, marry, is ’t.
But to my mind, though I am native here
And to the manner born, it is a custom
More honored in the breach than the observance.

Nor should this decision be about political correctness (mad or otherwise). Article 1 of the UDHR begins: All human beings are born free and equal in dignity and rights. Note the word dignity.
Blackface is a caricature of black people's physical appearance and as such impinges upon their dignity, irrespective of whether insult is intended or felt.