Just to establish some facts.......Blacking up as a disguise was banned by act of parliament in the 1700's so there is hard historical evidence that it was a recognised practice

This is an interesting statement and I never seen reference to this legislation before. Googling it suggests the GUEST poster is referring to the Black Act passed by Parliament on 26 April 1723.

Wikipedia tells us that this act was designed as a counter-measure to:-

the Blacks [who]gained their name from their habit of blacking their faces when undertaking poaching raids. They quickly demonstrated both "a calculated programme of action, and a conscious social resentment", and their activities led to the introduction of the Black Act to Parliament on 26 April 1723; it came into force on 27 May. The Act introduced the death penalty for over 50 criminal offences, including being found in a forest while disguised, and "no other single statute passed during the eighteenth century equalled [the Black Act] in severity, and none appointed the punishment of death in so many cases". Following a criminal law reform campaign in the early 19th century, it was largely repealed on 8 July 1823, when a reform bill introduced by Robert Peel came into force.

The societies of Bonfire Boys in my home town of Lewes all have a "Smugglers" section in their parades. Each member of that section will wear identical costumes - striped jumpers, blacked faces and identical caps pulled low over the forehead. These celebrate the various smugglers gangs which were very widespread in 18th century Sussex and for they dressed this way to facilitate disguise and to prevent individual identification. Those who benefited from poaching and smuggling were found in every class of society and the "Watch the wall my darling while the Gentlemen go by." attitude is well identified in Kipling's famous poem.
It seems entirely likely that there is blacked up morris was influenced by Mistrelsy after 1840 but is seems at least equally likely that dancers were influenced by the popular "criminals" who brought some sort of relief, employment and satisfaction at least 120 years previously.
The suggestion that black-faced dancers preceded the 1840 tour by the first American Minstrel parties seems to be very strong.