I've only heard of Mary Seacole out of the black Britons from history we're expected to know of.
I actually googled the musician and he has a relatively interesting story but it's also not at all surprising people don't know who he is today - he had one piece which was very popular called the Feast of Hiawatha which according to Google was played regularly until 1939 and then doesn't seem to have been revived. Seems he was much better regarded as a conductor.
Anyhow, historically this country's establishment has made it hard for black people to get famous until the 20th century, something that this academic surely knows. She's either naive or deliberately skewing her results for headlines by asking for names from a time when her top rankings include a Roman Governor!