Hi,
Rozzzie is right. You should try RICE in the first instance as if you haven't done too much damage it may well heal up of it's own accord.
You mention that it was black and blue so I'm assuming there was some bruising there. You can probably assume that there is at least a little inflammation but the problem with knees is that because we use them all the time, they don't often get the time to recover from injury.
If after a week it's no better, or if the pain gets dramatically worse for no apparent reason, then you should definitely see a doctor.
Take care,
Laura x