The lack of black baseball players is probably a combination of factors, but racism isn't likely to be one of them.
Baseball is hard to play in dense urban areas, it's a suburban/rural game, same with soccer to some extent, while basketball courts take up far less real estate than large open fields.Especially the Eastern cities which are denser and more built up. It would be interesting to see where white ball players come from.
In the southern states, where a large proportion of the black population still resides, football is king.
So it's less about exclusion than viable alternatives.
As far as ethnicity, as the son of an historian who writes multi-disciplinary history, I am always curious about people's ethnic background and history, I'm fascinated by the interplay of culture and conquest/immigration/emigration.