Can blood-building herbs cause mid-cycle bleeding?

I wouldn’t say it would never cause this, but it generally would be unlikely. What really matters, however, is what was the supporting diagnostic signs and symptoms you were using to begin taking ba zhen wan to begin with. If a practitioner recommended it, you should consult with them and if you simply chose it because it seemed like a good idea you should consult with a practitioner.

That said, there is always a chance of pregnancy (“implantation bleeding”) or that the herbs have led to some changes of the underlying hormonal balances and it will even out with the next cycle or two. If it’s just a few days I wouldn’t worry about it too much unless it starts happening every cycle.