Seems the big issue for you is the fact that this scientist works exclusively with Black subjects (or so it seems).

I agree that can look odd. But there are many scientists and research physicians who work with specific populations. I personally know a psychologists who only studies Black and Latino families. Why? Because she is interested in questions of family dynamics in these populations. But she is Black. And I've noticed that Black researchers doing research with subjects seems to arouse less suspicion. Is that your point?

If this researcher is primarily interested in the effects of lead poisoning, then it makes sense that most of his subjects are poor, black, live in urban neighborhoods and poor housing conditions. Lead poisoning is a BIG problem and primarily affects people whose live in old, poorly maintained buildings in former industrial areas in big cities.
Lead poisoning research is pretty much a poor black population issue. Do a search on this type of research and without trying, most subjects are persons of color, urban, and poor.

They are already exposed to those contaminants (lead in paint), the study wanted to know what is the impact of that exposure. The sewer sludge fertilizer met existing standards of fertilizer (akin to spreading manure on lawns) and the question was "will this fertilizer lower the concentration of lead subjects are already exposed to?"

Alarming people, specifically Black people, that here we go again with the Tuskegee Experiments, only interferes with the scientific and medical communties ability to resolve issues that affect people. In this case, the very people it tends to impact and harm the most - poor, marginalized citizens. Lead poisoning is a big problem - and it is pretty much a class issue.

The Urban Scientist
www.SciEdSociety.blogspot.com