I dont think winning Davy Back fight is enough.
Notice that no one from Xebec’s crew ever says they are staying out of respect for the Davy Back contract. They are there purely to fulfill their own interests. Last chapter, we even saw Xebec allowing his crewmates to leave. If you remember from Water 7, Zoro said that a crew which loses respect for its captain is doomed.
I dont see Zoro, a bushido believer, respecting Blackbeard's way of operating. Remember, Zoro didnt initially want to join Luffy, but did so because (1) Luffy saved his life, (2) he saw some good in Luffy (Coby informed that Luffy punched Helmeppo for trying to break his promis to Zoro), (3) he was drawn to Luffy's simplicity. I just cant see Zoro following Blackbeard. There is no way he would have part of Whitebeard's murder.
P.S. BTW
@Cruxroux good to see you back. Any publicity is good publicity!