Blackbeard has already eaten the strongest logia, and the strongest paramecia. His flag seems to be a three headed cerebrus, which could symbolize the strongest of the three types of fruits.
If kaido's devil fruit is actually a zoan, it is probably one of the most powerful if not the most powerful zoan fruit.
I think that luffy might be able to beat kaido, but blackbeard will somehow be able to take his devil fruit, meaning that he would literally be invincible.
If kaido's immortality (for lack of a better word) is Due to his devil's fruit, then blackbeard would pretty much be invincible. He would have strong offensive power due to whitebeard, devil fruit nullification due to his dark dark fruit, and an invincible body!
Sorry for any mistakes, on mobile.