I think (and please dont quote me on any of this) but if you are using a full color managed workflow, than the problem is that "black" in a color image isnt really "black" but a hue of a certain color. Desktop printers, have a hard time with black. Try printing a black and white image, and it wont be pure, it will have (at least on mine) a greenish hue to it, ever so slight, but noticeable.
I also notice color shifts when printing images in color that contain a good amount of black.
I have yet to come up with a perfect solution to this, but rest assured, there is nothing wrong with your printer, well nothing wrong in the "its not broken" sense.