Thanks, Cats. I think the quotes tell their own story. Compare
1599: revellers disguised by blackened faces
and
1780: Mylor [Truro] Mummers Play has the character Black Sambo in it

In the sixteenth century, nobody in England would make the connection from blackened faces to people with naturally dark skin, as the idea of people having dark skin was more or less unknown (witness the use of 'black' and 'brown' as adjectives for people, referring to their hair colour).

In the eighteenth century, the connection was there to be made, but in most cases there would be nobody watching who was likely to take offence (least of all in Cornwall).

200 years later, lots of people would make the connection, and quite a few of them would take offence.