Everyone will tell you that it's the Imaging Unit - happy to say that's not true. If it was the imaging unit the colors would not come out right either. I bet if you print a test page that all the colors come out fine across the page. It's only the black.|
Also - if you replace the black toner cartridge, or reinstall your printer and drivers like some people will tell you - still no results.
The problem is in the black toner cartridge PATH inside the printer. Something has jammed it up a bit. Try taking out the black cartridge and blowing firmly into the small hole at the bottom of the spout that looks to have a sponge inside. Use a straw and make sure your air blown is dry. Then lightly tap the cartridge a few times. Then use the straw and once again blow very firmly inside the receiving path (inside the printer) for the spout on the cartridge. Make sure you are blowing into the spout path at the bottom - not the gear drive path in the middle - as that will not do any good.
Replace the cartridge, warm up and print another black and white page. You should see that the faded out section is gone or diminished in size. If it is not gone, simply repeat the above steps until it is gone. You will see that the faded out strip gets smaller and smaller each time until it's gone. I had to repeat five times, but eventually got it corrected. Now prints like brand new so don't get discouraged.