Black Lava Bubble
From the Super Mario Wiki
Black Lava Bubbles are black colored versions of Lava Bubbles that appear in Paper Mario: Color Splash. They only appear in Black Bowser's Castle during the battle with Black Bowser, where they are formed from the black paint that drops from him and assist him in his battle against Mario and Huey. Depending on how much damage Black Bowser takes, the Black Lava Bubbles can vary in size when summoned. Only three can appear at the same time before they return to Black Bowser.