It doesn't seem to affect my damage output at all, actually. I noticed Kain's jumps with darkness active didn't do any more damage than twice of his normal atk; figured this could be because darkness doesn't stack with jump, but I tested and it doesn't enhance his dmg with regular attacks either. Could this be because darkness actually adds dark elemental damage ... there is no such thing in the game, right? (I was up against Dark Bahamut so if it is indeed dark dmg it makes sense that it doesn't add any dmg in that particular fight.)
I could be bounded in a nutshell and count myself a king of infinite space -- were it not that I have bad dreams ...
I've noticed the same thing, with the Darkness augment not adding extra damage when Kain jumps. What weapon did you have equipped? I think I had the Holy Lance equipped, so maybe that's why it didn't work for me.
One thing that can be very annoying is how quickly it runs out though, especially if playing with maximum combat speed. Spell/ability animations seem to eat through the time, so on fast combat speed with a couple long animations from other party members - darkness can wear off before executing a single attack(probably even more likely when trying to use with Jump.
So check to see if it just wore off rather than not working entirely.