Printing Errors or Lights & Stuck Print Jobs

The last few days I've been having issues with my HP 6520 printer and printing black ink.  My first issues started when I printed a document and the text came out printed a very light gray.  The cartridge was almost empty anyway so I changed it for a new HP cartridge, I've only ever used HP cartridges.  I then tried to make a copy of a black text document but the printer would not print anything, it went throught the cycle of printing on the paper but no ink was applied.  I then tried to clean the printhead multiple times but everytime the print quality diagnostic page would print the text would show up in black and the color block boxes print as black but there was no black ink printed in the black color box.  I then tried putting a damp paper towel in the printer and moving the printhead back and forth over it to try and clean ink residue from the printhead.  The printer still would not print black.  I left the printer alone for a day and then tried it again yesterday, I printed a text document and the black worked as normal but I then printed a picture and halfway through printing the picture the black turned to blue on the picture.  I tried today to print a document and all the black text came out as blue.  The vent on the cartridge is open and there is black ink from the cartridge on the 'screen' below the cartridge going into the printhead. I've tried many suggestions in the forum and now I don't know what else to do. Thanks in advance for any help.