Since you already replaced the black toner, I suggest also trying to remove and reinstall the black imaging unit. If you are still having issues after trying these steps, I suggest that you call into our support department for assistance @ 1-800-821-2797.
We replaced the Fuser and Maintenance Kit (transfer kit) on our Xerox Versalink C405 (due to the printer telling us to replace them), before we replaced the parts it all worked fine. The next day after replacing the parts the black printing staretd to fade, the colour was fine, so we thought it might be the balck toner so we replaced this and still no change, 3 days on our black print is that faded we can hardly see it. any help on this please as we are wonder if any of the parts could be faulty which we installed. it is the same if we print or copy.