This is Scratch Cat. He is the mascot of Scratch, as well as the default sprite in scratch. When a user starts a new project, the Scratch Cat is included and the user must remove it if they do not intend to use it. Many users first program is the Scratch Cat doing something simple like running or chasing the mouse or bouncing off the edge etc.
*Note: The other shades of black, are also actually various dark greys.