Arelith

It's not a bug at all, considering that CoT was never meant to combine with BG, but just to throw this out there: tested, and BG's Smite good doesn't stack with CoT levels for the damage calculation. This means no love for evil smiter builds. It's probably a silly build anyway, but it'd be a service for the memers out there.

So, smite infidel (the actual class feature of divine champions) can stack with smite good or smite evil, but whether or not it does is dependent on whether or not the target also worships a different deity from you. Typically this isn't that hard as most Good and Evil characters don't frequently worship the same deities, with some errant LN deities as the primary exceptions (Jergal, Kelemvor, Helm, the previous Mystra, and the like). Otherwise it normally fires the damage of whichever one applies.

The benefit to CoT levels with BG levels is that it makes it much easier to accumulate a higher number of Great Smite feats, although this provides a bit of diminishing returns with Smite Good until it's set up to factor in the target's patron deity as well. Builds with lots of blackguard levels tend to be feat-starved, so this is arguably still a win situation for an evil smite build- someone better than me with the numbers could probably optimize a specific point at which the extra great smite feats you get aren't worth the damage multiplier you're giving up from less BG levels. I suspect if you were going to all in on that route, you'd probably want a scimitar for reliability.

I also suspect that if you went scythe and critted a smite you'd literally delete any build on the server other than palemasters, but that's what I'd call cool as hell, not reliable.