Canon iX6850 - PGBK printing fuzzy

Hi

I have a Canon Pixma iX6850 printer. I have just replaced all of the cartridges but the PGBK ink is printing fuzzy, almost like it has a shadow. I've tried the nozzle check, cleaning and aligning the print heads in canon utilities but it doesn't seem to have made much of a difference. Is there anything else anyone can suggest I try?

Thanks