The Opies took their material as they found it. Most of their collecting work was done in the post-war years, when there weren't very many Black Britons as compared to today (bear in mind that Peter Opie was born in 1918 and died in 1982; Iona was born in 1923); on the whole, they were more interested in the "separate" culture that children maintain among themselves than in questions of separateness based on race.

That's probably more of an issue now, given that many urban communities are predominantly Afro-Caribbean, Hindu Asian, Muslim Asian and so on; but that's a recent demographic development. The British experience is very different from the American. Certainly there is room for new research in view of these changes, but the Opies' work pre-dates most of that and was based on how things were at the time, not how they are now.