So, Melanism in game is literally just the opposite of Albinism/Achromism

While that IS basically what it is, it's not just being ALL BLACK with no markings

Albinism is just a complete lack of pigment. Melanism isn't just the inverse. It's not just ALL PIGMENT. Because, that doesn't really make sense. There's not a scale of "no pigment to full pigment" There is no "full pigment".

Melanism isn't just being all black. Melanistic animals will still have markings, just like non-mutated ones. While I do believe some animals have it more then others, to the point where they will be all black, most aren't that way, and you can still see their markings.
eg: https://ichef.bbci.co.uk/news/976/cpsprodpb/2FE8/production/_130646221_rarejaguarinkaarrivesatchesterzoo-23.jpg

Since I know many people still like the current melanism, my suggestion is to have a variant that works exactly the same, the only difference being that the overlay is just slightly transparent so that way the markings still show. The markings might be recolored so they are all a dark black, but that's not needed if it makes it too complicated.

Also, I have another mutation idea I may be posting soon loolllll