I actually realized that the damage came from when I stood the gun upright and I swabbed the barrel between shots and as I pulled the patch out of the barrel some of the residue was falling off the patch and spilling off the edge of the barrel and onto my lens which was facing up.
Another good reason to avoid sticking a cleaning rod down the muzzle.
As far as the original question, if you're going to shoot a muzzleloader with a scope on it, chances are it'll be a modern in-line rifle shooting sabots or modern ML bullets and using a 209 primer. There's no reason to use BP in that kind of rig, it's hard to find and you'd be better off using a BP substitute like Blackhorn 209 or Triple 7.
The corrosive properties of BP come from the sulfur--it combines with moisture to form sulfuric acid. So if you want to use it, check with the optics manufacturer and see what they say about that.