My bet is that it's your printer. If there is no other color in your document, set it to print as greyscale only, not color composite. I'm guessing that you're printing to a desktop ink jet or something, not a color laser printer? If that's the case, your printer probably uses all the colors rather than just black to print greys, even though the image itself is set to black only. I don't think there's much you can do about that, but I could be wrong.