If you're seeing all areas of black spreading on your printouts, I may have a solution.
I just stumbled upon this:
When printing to "US Letter " paper size, all black images and type look good.
When printing to "US Letter Borderless" paper size, all black images and type are spread (widened) in one direction. Regular type looks like bold type. Squares with a 1/2-point black outlines print as if they have 1/2-point thickness top and bottom, but 1-point thickness right and left (or vice-versa depending upon whether printing Landscape or Portrait).
The printouts look almost like the black ink areas were laid down twice, but with a slight shift between the two passes. All areas that do not use black ink look fine.
Solution 1: don't use: "US Letter Borderless" paper size.
Solution 2: replace use of Black in your file with a mixed ink of 100% Cyan + 100% Magenta + 100% Yellow. This yields a dark gray (not quite black) but allows you to still print borderless.
Hope this helps.
--Nathan