Why is Bowser black in Bowser’s fury?

Why is Bowser black in Bowser’s fury?
Similarly to Black Bowser from Paper Mario: Color Splash, Bowser enters this form when he is corrupted by black goop, which causes his skin to become entirely black in color, complete with glowing yellow eyes without pupils.

Fury Bowser

Latest appearance
Super Smash Bros. Ultimate (version 11.0.1, spirit cameo) (2021)

Similarly to Black Bowser from Paper Mario: Color Splash, Bowser enters this form when he is corrupted by black goop, which causes his skin to become entirely black in color, complete with glowing yellow eyes without pupils.

Fury Bowser
Latest appearance Super Smash Bros. Ultimate (version 11.0.1, spirit cameo) (2021)
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