not sure there is a name for them.
but you mean groups like ..
"
Liverpool-born Blacks are people of Black African ancestry born in the city of
Liverpool. Liverpool has the United Kingdom's oldest and longest established black community, going back several generations.
[1] Liverpool's black community is also unusual among those in the United Kingdom, as the Liverpool-born Black British community often constitute a category distinct from later African and Afro-Caribbean migrants.
[2]"
"
Dating to the 1730s, the Black community of Liverpool is Britain's oldest, with some Liverpudlians being able to trace their black heritage for as many as
ten generations."
en.wikipedia.org