I'm surprised nobody has referred to Trish Bater's dissertation "'Blacking Up': English Folk Traditions and Changing Perceptions about Black People in England", here: http://etheses.whiterose.ac.uk/4181/1/MPhil_upload.pdf.

In her executive summary Trish says:

"It is concluded that here is a long history of blacking up in England and that current blackface practice is not intentionally associated with Victorian 'nigger minstrels'; however, minstrelsy was so popular that its influence cannot be discounted. It is also suggested that the folk process has always absorbed and reflected influences from popular culture, and continues to do so; and that in an area of strong opinion, no definitive answers can be given that will satisfy everyone."

In the dissertation she deals with many of the points and arguments that have been mentioned in this thread - I'd advise anybody to read her well-researched, well-written analysis before making any more wild generalisations!