Male cats spray urine to mark their territory. The urine contains pheromones which leave scents as messages for other cats.

A black cat spraying onto a white cat may be code, as Azizi says, for aggression by black people against white people (as in "your ass is mine")... or perhaps it was a way of saying that, though white slaveholders owned blacks, the blacks' work accounted for the whites' wealth so, in a sense, the blacks controlled the whites. Either way, the "piddle" marked the white as being the territory of the black.

...or it could have been a code for a sexual reference: a black man claiming a white woman as his own.