Does a kneecap actually bruise inside? i thought it was a lump of solid cartilage?

Yes - in a sense: The kneecap is a bone with a layer of cartilage on the deep surface. With trauma the cartiage can be damaged and the underlying bone can get a "bruise" which is known as a bone contusion

Kneecap bruise: A bruised kneecap occurs from a direct injury to a kneecap, which usually happens from falling onto a knee or by being hit by an object. Bruises can last from days to months, with the bone bruise being the most severe and painful.