Is Black Death the first African American heavy metal band?

Yes, but only if you specify they were "the first doommetal band."

They were among the first heavy bands that incorporated metal as their whole sound, but other bands (such as Led Zeppelin) had had played songs that could be considered heavy metal

I think the main difference is basically subject matter and sound. Led Zeppelin are grouped in as heavy metal but in reality are a blues based band,not metal,they say this themselves. Heavy metal,particularly Black Sabbath wrote songs about the " dark " side of life and things in general,as opposed to " flower power" at the time(late 60's). Their unyielding,sonic sound was an answer to the blues based,melodic style of the times.