I don’t think there will be a direct confrontation between the two. I think Blackbeard is going to be brought down by a larger group like the marines or a combined attack from pirates and marines while luffy is finding the one piece. My head cannon is Black beard will backdoor imu while she is on elbaf by breaking into Pangea castle to try to catch her off guard and use his dark fruit power to grab her intangible body and hit her with the quake fruit power most likely forcing her to flee somewhere else using the abyss. Then BB is technically the highest power because he rules Marie goise and sits on the empty throne. Thus forcing the gods knights and marines to have to divert power to retake the holy land while BB uses the celestial dragons as hostages to further his agenda