It's very possible that I'm not understanding you properly, you seem to have a few different systems in mind. You said you want creatures to be "almost black" outside of lit areas. If they are, in truth, detectable, and there's no cost to the player for scanning the screen with the "look" function, then the net gameplay effect is increasing the duration of every turn. Not that you can't do some pretty things with full color ASCII, Brogue is incredible.
I brought it up because I've been learning a lot about design and presentation of information, and the general consensus is that color is not a good way to communicate information. I've also played with lighting-based color manipulation in some small libtcod projects, and it seems to be more trouble than it's worth. You have to consider how color blending obscures information, how it changes on different displays, and how the color-blind interpret it. If ToME can't easily support this, you may waste a lot of time with an effect you end up ditching.
Sorry about all the parentheses (sometimes I like to clarify things).