Blackbeard's physical strength is underrated, he's much more powerful when it comes to physical strength if they get in a brawl Blackbeard would destroy Doflamingo.
And Blackbeard has all the tools to counter Overheat, Parasite and 5 colored-strings.
If Blackbeard can stop Ace's Entei then he can stop Overheat.
Blackbeard also has enough durability to take an quake in his face, and Shockwaves from Sengoku. So it's more than likely that Blackbeard outlasts Doflamingo in that aspect, too.