Black Bowser (referred simply as Bowser in-game) is the main villain in Paper Mario: Color Splash.
When Bowser came to Port Prisma, he saw the Prisma Fountain with its rainbow of colors and decided he wanted a rainbow colored shell. He jumped in and mixed all of the paint together while jumping around and created Black Paint. Then the Black Paint possessed him, and he became Black Bowser. He flings the Big Paint Stars far away and shortly afterwards, orders Slurp Guys to suck out the paint of almost everything there.
After Mario and Huey return the Red Big Paint Star to Port Prisma, Black Bowser appears on the highest spire of the Red District. He delivers an ominous message that the dark gift of black paint will be shared with the world soon. He then leaves Port Prisma by jumping onto an airship, which carries Princess Peach away to Black Bowser's Castle.
He later reappears in the memories given by the Big Paint Stars, telling how he became the monster he was. His last known appearance is at the end of the final boss, where Mario beats the black paint out of him.