I would say that 99% of CMYK can be near-perfectly reproduced in RGB (when you color digitally).

“Rich Blacks”? I assumed you mean something darker than your typical 100 K? RGB “Black” is Darker CMYK Black. Mainly because it is easier to make a black in light (RGB, turn all colors off), than it is in CYMK (inks).

“Black” on RGB on Photoshop is a crazy set of numbers between the CMYK sliders. From our Professional Coloring episode (http://www.webcastbeacon.com/webcomic-141/), Melissa uses a 100 K Line Art layer in Multiplt, over a Normal “under color” Layer 60 C, 40 M, 40 Y, 100 K (2px contraction) Line Art.

This is what I was talking about for accounting for “plate shift”, thought Melissa also told me that it should still help give crisper lines in Digital Printing too.

So with that in mind, I would say a “rich black” in CMYK would be 60-40-40-100. At least that is what Melissa, whom has worked as a professional comic book colorist, has suggested. Melissa warned that mixing too many colors in, high percent, together, can give you “muddy” looking colors. This can vary by printer of course, but she does not recommend doing an 100, 100, 100, 100 for a “rich black”. Basically because you are telling 4 colors to try and fulling place themselves onto the same area of paper. And if you have thin and/or not very absorbent paper (to thick and/or very absorbable paper), your results could vary drastically.

At the end of the day, it might come right down to “well, maybe only us artists will REALLY notice the difference.” But I don’t think there have been many tests and comparisons with this sort of thing, for what is available to us.