MLB teams once did the work of developing young players in their system. Not long after Curt Flood won the right of free agency for players and contracts began to rise, things started to change.
sports.yahoo.com
This whole thing is kinda overblown. There's plenty of afro-caribbean players in this upcoming series.
I think the lack of African-American players can just be explained by the fact they're not gravitating as much to the sport as they were a couple decades ago. Baseball these days for kids isn't just picking up a bat and ball and going to play. It's an organized little league, travel league, rec that all costs time and money. It's turning into another rich people sport in North America. Hispanic countries don't seem to have that problem.
Lot easier for a kid (be it black, white, Hispanic, whatever) to pick up a basketball and shoot around with their friends or play touch football.
Not to mention the sport as a whole has kinda crashed in popularity compared to basketball and football.