none of this is terribly relevant in the context of DEADEUS, which is a ROM that will look different based on the emulator or hardware it's played on
but you are half-correct. black was a color that the original Game Boy and Game Boy Pocket used in some internal capacity, but they could not really display it as black due to the limitations of their non-backlit passive matrix screens. the end result was that the entire screen had too strong of a green or off-gray tint for truly black pixels to exist in any well-lit environment
on the other hand, the game boy color had 12 palettes which it could use for regular GB cartridges. 11 of those contain the unmistakable, inky #000000 black