1-bit pixel art. They didn't have any of that fancy stuff like anti-aliasing and color. No, back then, they had black pixels, and they had .. white pixels.
Homage to an old game called "dark castle", which, although it had some terrible ports to consoles, really was only any good on either the amiga, or the mac where it was originally developed. (Notably, the mac had really tight 72dpi resolution, so you could dither and it would actually blend just like it does on a modern VGA screen). At the time it was a wicked action-platformer with rpg elements where you, the valiant prince Duncan, went into the titular Dark Castle and took on the Black Knight. It was also really hard, with fixed-jumps and instadeath like Ghouls 'n Ghosts, but it was really ahead of its time, and definitely inspired me as a kid.
And yes, the dark knight had precisely this ridiculous design. He was a walking tin can.