Don't be so sure. Look at a color printout closely. See if the black ink in the color printout is actually behaving the same way.

This might be tough, because the point of a four-color additive printing process is that if you combine Cyan, Magenta, and Yellow at 100 percent strength, you get a color that is nearly black anyway, and the black ink is only there to reinforce it and make for richer dark shades.

Anyway, if you're sure that's not it, if you're sure the black ink head is behaving correctly for color prints, then check and see if there is some kind of a "paper" setting in the printer driver that controls the type of paper. If you put in glossy color-print paper, and select glossy paper in printer driver, that's a different way of applying the ink than plain paper printing black and white. Maybe it's just a setting in the printer driver.

And, of course, make sure you're not putting special one-sided paper into the printer upside down...