So is the black cat helping or manipulating Coraline? The very end of the movie might provide an answer.

After some adventuring, Coraline saves the ghosts by finding their eyes and destroying Beldam’s constructed world. She then throws the key to the Other World (which represents her psyche) down the magical well she found while water-witching. Coraline then happily goes back to her real world and tries to make the best of real life. Her family organizes a party in their garden – the same garden that was made to look like Coraline in the Other World. As the view of the garden pans out, observant viewers might realize something odd.