I think this is very plausible. See this thread, and specially notice the posts by @Harvey . It seems that not only dye black can be affected by the fuzzy printing problem, but other dye colours can be affected as well.
Only the photo black has a visible defect in the nozzle pattern. The other colors band look perfect even when magnified, but not on their respective blocks. Remember the blocks in every color is a mix of all of the other colors. The reason for the not perfect blocks is because the photo black is damaged and when its nozzles print in other color blocks they are printed bad. Even with perfect dots on the color bands prints are not good.
Upper section MP620, bottom MG6120 with fuzzy black. This was printed in the same sheet with same ink for both printers.