Not only that, but Duane Jones got the role solely on merit. The part was not written with an African-American (or a minority) in mind, but Romero decided after

Not only that, but Duane Jones got the role solely on merit. The part was not written with an African-American (or a minority) in mind, but Romero decided after the fact not to update the script in any way, which gives the film this odd colour-blind/progressive trait for that time (1968)... (Mild spoiler below) ...And then post-filming but before its release, Dr Martin Luther King Jr was shot, giving the film's treatment of Jones character an unexpected tone in context. Romero mentioned this in one of his interviews to illustrate another way in which the film was shocking to its audience (beside the then never-seen-before level of gore in the movie -- something that has gone mainstream since then and may get lost when watching the movie).