Just after I finished writing the previous post, I found a letter published in the 15 January 2004 issue of Country Life in which Jane Clark suggests that the black servant in a portrait of the Earl of Burlington (at Chatsworth) may be a Jacobite symbol. Both Charles II and James III, the ‘Old Pretender’, were sometimes referred to as ‘the black boy’ - originally a reference to Charles II’s dark complexion.
Personally I think this explanation is a bit unlikely, not only because it has too much of The Da Vinci Code about it, but also because most of the depictions of black servants in paintings are clearly portraits of real people, rather than mere symbols.
That said, black servants were definitely being objectified in the sense that they were regarded as something of a fashion accessory in the late seventeenth and early eighteenth century.
Although they might have personalities, black slaves were still chattels, to be bought and sold like any other exotic accoutrement.
That is why the 1730s portrait of the black coachboy at Erddig is relatively exceptional. He is not shown as an exotic rarity adding lustre to the portrait of his masters, but just as himself, in his professional livery and with a tool of his trade.