Blackbeard doesn't fight unnecessary battles, where the reward isn't worth the risk.
He stood nothing to gain by fighting the Red Haired Pirates during Marineford.
Why fight Akainu over a Ship?
Why risk fighting Rayleigh, the Marines + Seraphims, and the entirety of Amazon Lily for Boa's Devil Fruit, when it would have cost him two crew members right off the bat?
If it is something detrimental to his overall goal, he will lay it all on the line like he did for the Gura.
Blackbeard will be a Conquerer.