This is very interesting, and don't know why I never thought to mention it here. I have noticed that when I print black it takes forever to dry (or on paper it never seems to dry), I have always let the RIP determine what black was, and sample the color picker to RGB 0,0,0 which is c-75, m-68, y-67, k-90. Perhaps this is my issue.
For me, printing on banners every color but black is very colorfast, but black is easy to scratch - i can leave a fingerprint in black prints half an hour after they print... every other color seems dry almost instantly... As of late, I have been backing off on ink saturation if i know there's a lot of dark areas to compensate and it seems to work okay. Perhaps I should try some of these combinations instead. I also work in a CMYK colorspace, not RGB.