Why isn't Blackbeard dead? It's a known fact that Blackbeard had a fight with Magellan.
Magellan poisoned Blackbeard and all of his crew. 

They're seen covered in Magellan's poison after the scene below.

Why aren't they dead and how did they (also) escaped Impel Down?
That's because Shiryu gave them the antidote before joining their crew.



It happen in episode 452 of the anime and in chapter 549 of the manga.