That's what the lightRadius does, unless you're talking about ambient lighting, then, yeah, that's controlled by Let There Be Darkness, Darker XXX, etc. I think you have to change it in the cells. That's really odd though, since Cava Obscura drops the settings to 10% of the original instead of doing a black out. Have you confirmed whether or not its changes are actually being "seen" in the game with TES4View? Maybe the changes are not being imported properly or something. I used to have Cava Obscura and Ambient Dungeons in my LO. I still have them both installed, but I haven't reactivated the latter quite yet. I'll be doing that soon I think. I do remember caves being basically pitch black, but torches still seemed to be providing nearly sufficient lighting. (I definitely need to up the torch radius in DLT's INI though.)
--Tomlong75210
Edit: Maybe you should post your load order too, but do check it in TES4View to make sure that what you are expecting to appear in game does. If you aren't able to tweak other things to get the affect you want, maybe you should drop Cava Obscura. I wonder where wrinklyninja is. He'd certainly have some answers or suggestions...
*Summoned*
Yes, Cava Obscura makes things dark. I use the OBSE DLT mod to increase the radius of my torch, I think I double it or something. You're right that the torches aren't enough at their default radii as they were put to that because there was plenty of ambient light to see by, and the whole light attenuation thing in oblivion isn't spectacular, so light given off by torches doesn't really give any 'hints' of things outwith the set radius, hence the need to increase that.
I too enjoy the sneak benefit - I don't think it's sensible for someone to ping you on their radar if they're standing in a pitch black area and you're moving around barefoot (since the rest of your armor doesn't count... steel plate clearly doesn't clank), I sure wouldn't know if someone was right next to me and holding their breath. :shrug: