I was at a talk on Black Invisibility in Art, which started with Dabydeen's work and itemised various occasions when critics and curators have wilfully ignored the presence of [often marginal] Black figures when analysing old master paintings - e.g. the right-hand female attendant in Titian's Diana & Actaeon.
Someone in the seminar then cited the fact that the National Portrait Gallery's Anti-Slavery Trail for the 2007 anniversary [devised by Caroline Bressey] had been taken down - as an example of brief, temporary, token visibility.
So I should put in a plug for the current display featuring William Cuffay, drawn by Paul Dowling when they were both in Newgate Prison awaiting transportation for Chartist agitation. As far as I know. it's the only portrait of Cuffay - who settled and died in Tasmania - and was presumably reproduced as a lithograph for sale to sympathizers - i.e. as a political act. Like most works on paper it won't be on show permanently, but it is permanently visible on the NPG website - search for Cuffay.