Three points arising from some of the interesting contributions to this thread. -

1] 1840 and the first visit of the Ethiopian Serenaders is quite significant date. Let's look at a few significant events before that -
* 1792 Burns song/poem The Slave's Lament was published and had a huge impact on thinking. That Africans had the same feelings, fears, sense of loss etc. was revolutionary at that time. Burns was seen as the friend of humanity and an enemy of injustice or oppression and part of a growing movement particularly strong amongst Quakers and some other non-conformist churches
* 1807, the Abolition of the Slave Trade Act although Wilberforce had been campaigning about this for decades and first introduced an act in 1789 on the subject. His earlier efforts failed to get a majority.
* The next big step was in 1833 the Abolition of Slavery bill (in the British Empire) was passed - 3 days before Wilberforce died.

Great! British people had been in the forefront of Abolitionist thinking and action - but there was still a heavy residual guilt. Much of Britain's economy was still dependent on the existance of slavery. How many jobs existed because the the import of slave produced raw sugar, cotton etc. etc?
When the American minstrel shows came to the UK from 1840 with their message of the 'happy slave' it was just what the public needed to assuage this guilt. Decades ago I interviewed or spoke informally to many surviving old singers and musicians. Many of them remembered blacking up to sing in village concerts.

2] Some discussion has taken place about the existence of black faced dancers before the coming of Minstrelsy. 1840 could also be given as a date for the arrival of the popular press and photography in this country. Any social historian will tell you that any research to gain reliable information becomes much more difficult. As far as morris sides are concerned, it is difficult to establish just how widespread this was, never mind what music, costumes. dance figures were, never mind whether dancers blacked up or not.

3] This point is on a more personal level. As both a mummer and a morris man in my younger days, I have been challenged to think about things associated with it. Here are two questions that I asked myself.
What is your attitude towards black-faced dancers? Still fairly relaxed.
Would you ever black-up yourself? I never have and I never would.