Prestonpans Dunn's Black - ring anyone's bell?

[Mountainpeople]

Apologies in advance because I'm well aware that calling someone 'A Black' - as in their colour - is most definitely not acceptable nowadays, but my father's grandmother, Mary Agnes Dunn, born 1863, died 1940, married Andrew Gray who worked in the Prestonpans Brickworks. Andrew was born in 1858, and died young in 1895. Being in reduced circumstances, my Grt Grandmother ran a shop in Prestonpans, but she also took in a lodger - a black student who was studying medicine at Edinburgh University. He was, apparently, referred to as 'Dunn's Black' by the locals, and I have a lovely photograph of him in his graduation gown, wearing what looks very like a medal which would suggest he did very well in his exams. I know no more about him than that, but I would very much like to give the photograph to his descendants if he has any, so if anyone recognises this person, and can give me a name, I will take it from there.