Yes, you can still cut it back, although that is something that is normally done much earlier in the spring or even late winter.

And depending on where in zone 8 you are located, you might want to reconsider growing this shrub entirely. Butterfly bushes are becoming increasingly invasive in warmer zones and are actually listed as an invasive species in several states and prohibited from sale, unless you plant a sterile cultivar. Black Knight is one of the most prolific and fertile self seeders.

Your bush is quite close to the house... does it receive full sun?

Are you watering it? If so I'd stop. It looks as if it's over watered and fungussy.

And are you sure of the id? It doesn't look dark enough for BK.