I've just recently come across the claim of pre-Columbian indigenous American blacks. It seems to claim that there were tribes of "black indians" in California and other places when Europeans arrived.
How credible are these claims and what is the source material they're working from?
There is a series of posts on the faq that address this topic in particular. To be quite honest, this theory is not credible at all. Ivan van Sertima's They Came Before Columbus has been heavily scrutinized for making making the claim that Nubians advanced Mesoamerican indigenous groups, thus why the Olmec heads have "African" features and they have pyramids. This work has become one of the major corner stones of Afrocentric pseudohistory and has definitely inspired other "historians" to support his claims.
It is not credible at all. This past posts cover the topic comprehensively: