Yes, I experience this awkwardness too, sometimes. It depends on the situation, but when I feel that I can't get past the b in time and the circumstances allow it, I just give them a warm smile, that tends to be ok. You aknowlegde the sneeze, but you don't have to speak. When I have headphones in (on the bus e. g.) I just pretend I didn't hear it. If I have to say something, I just make a noise that sounds vaguely like two syllables and the other person normally understands that as a "bless you".