Did you install the new black cartridge before the previous one ran out completely? If not, the magenta may have seeped into the black's print head and caused the discoloration. did you use one of those inkjet refill kits for magenta recently? You may have overfilled that cartridge and it is seeping. In either case, cleaning the print heads might fix it, but if you have printed 50 pages already, i don't think cleaning the print head will help, either. It may just be a bad cartridge. If it is an HP branded cartridge, you can try contacting them directly or just attempt to return the cartidge to your place of purchase for an exchange. If it is 3rd party, I would think it probably is a problem with the ink used by that company. You can contact them directly, too, or chalk it up to a bad investment and replace the cartridge again.