"In today's context it should not be used. Does that unfortunate use illuminate whether the blacking up was a reflection of "nigger minstrel" shows, or a reflection of earlier guising?"

Sharp mentions blacking up, but as something that is uncommon at the time of writing and which is largely consigned to the past (though he makes it clear that his notions of it having been once widespread are conjecture). He also doesn't say how long ago this practice might have been common - and as minstrelsy had entered into British popular culture as much as 70 years before the time of writing, it is very difficult to say whether Sharp refers to a time within the past 70 years, or to an earlier time.

However, as Derek's article said, "There are references to people blacking up as a form of disguise in popular custom, although in Heaney and Forrest's book 'Annals of Early Morris', there is only one reference to black-faced morris dancers in the period they studied (up to 1750), and that is from the mid-sixteenth century."

So up to 1750, blacking up would appear not to have been a widespread custom within morris; but Sharp talks about it possibly being widespread in the past (some time prior to 1912). It is very difficult to say whether border or Anglian morris had a blacking up tradition prior to minstrelsy, at least from this evidence.