problem!

i just finished photographing my friends skateboarding today and i noticed that all of the pictures have a black blurr on the same part of the picture, i cleaned the lens and tested it and it was still there. i believe that maybe some of the pixels are getting messed up. i was curious if anyone here could help me figure it out? and tell me if it is fixable. here is a photo as an example. the kid at the top of the stairs in the grey shirt has the black blurr at the bottom of his shirt.

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What camera is it.

If it's a digital SLR (which it probably is if you're posting in this forum), it's probably dust on the sensor. If you can find the last two issues of Photoshop User Magazine, they have a great article on spotting dust and fixing it. A simpler (but slightly less effective) way is to take a photo of a white piece of paper. Then open it in photoshop and use auto-levels and it will give you a pretty good idea of where the dust is.
Chip

the camera is a nikon d1 and as far as i can tell there is a black spot on the camera's sensor. i believe it is just dirt but i gave up and im just going to send it to nikon for cleaning, thanks for the advice and help!