AOG
The tradition of minstrels/blacking up is offensive, not because black people are ashamed of their skin, but because of the history of the act. It was never done with respect. Minstrels were white people acting like 'foolish blacks'.
Blackface is much more than just dark makeup used to enhance a costume.
Its American origins can be traced to minstrel shows. In the mid to late nineteenth century, white actors would routinely use black grease paint on their faces when depicting plantation slaves and free blacks on stage.
To be clear, these weren't flattering representations.
At all.
Taking place against the backdrop of a society that systematically mistreated and dehumanized black people, they were mocking portrayals that reinforced the idea that African-Americans were inferior in every way.
The blackface caricatures that were staples of Minstrelsy (think: Mammy, Uncle Tom, Buck, and Jezebel) took a firm hold in the American imagination, and carried over into other mediums of entertainment.
The Morris Dancers however are a completely different story as far as I can tell, and people need to understand that they are not portraying black people at all. As far as I'm aware it's something to do with miners isn't it?