I know that translator's note at the end is probably older than Twitter, but I'm pretty sure the reason this series only got three volumes is because it's far too similar to Berserk.
And it was published in the same magazine as Berserk.
So when the volumes went on sale people ignored them in favour of an older, better fleshed out series that already had a cult following. Simple as that.
Pretty sure the homogeneous Japan would see no controversy in a black protagonist. If anything, Blaster Knuckle would've been appealing due to its resemblances to American cinema.
You know, the good ol' exotic factor!