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Based on the length of the mark and that it is black only, it would have to be the black drum. Are you saying you need to know how to change it? Or are you able tondo that if you get the replacement?
This could just be residual toner from the old one - have you tried blowing it out with compressed air?
You are free to do that... but if it's anything but the toner/drum, then it should clear up with time. If you think it's just not fusing to the paper but it's being laid down on the paper in the right place, then it could be the fuser roller - or more likely the fuser itself. But that should not affect only black toner.