He looks like himself, but has black paint covering all of his body and yellow eyes. The black paint is constantly dripping off his hands and shell.
Bowser came to Port Prisma one day and spotted the Prism Fountain. He thought the idea of a rainbow-colored shell was cool, so he jumped into the center of the Fountain and spun his shell around. But instead of getting rainbow paint, he created black paint. He was having fun until the black paint became sentient and started to consume him. Bowser tried to resist, but he gave in to the dark influence and merged with it, transforming into Black Paint Bowser. He had one goal in mind: to paint the entire world in black and envelope it in eternal despair. He equipped most of his Shy Guys with straws that could suck up paint from objects. He transported all that paint to his floating castle, mixed it into black paint in the factory, then filled Banzai Bills with the toxic substance. He might have also sent his Koopalings to collect the Big Paint Stars because he knew that they could supply him with all the paint he needed to make more black paint. His plan was stomped and tromped by Mario, who faced off with him in his castle at the end of the game. When he lost a substantial amount of black paint, he used the rest of it in his "last breath." Huey absorbed the black paint, and Bowser returned to normal.