Unelss I am mistaken, I'm not sure if we ever really know what Jack beleives about Blackbeard. Obviously, once he actually sees Blackbeard and all of his wrath, he understands that he is in fact the leader of the ship they are on. However, he has some doubts up to that point that are rather hard to follow, but Jack has always been kind of like that. You never really know what's going on in that noggin of his. When he waks up on the Queen Anne's Revenge, it's Scrum, a fellow crew member, who informs him of the ship that he is now working on. Scrum: "'Scuse me, Captain Sparrow, sir. I be right honored to welcome you aboard our world-renowned vessel of infamy—Queen Anne's Revenge." Jack, being a pirate from the inside out, and being any sailor really during that time period, knows what infamous captain sails the Queen Anne's Revenge.
He gives Scrum this super scary and strange face. The camera zooms in on his face as he growls: "Blackbeard." But then he seems to be rather confused about the whole thing, especially when it comes to his relationship with Angelica. Is he or isn't he his daughter? Angelica: "Long-lost. Recently found. Who loves her dear papa with all her soul." She gives hikm this cutesy little shrug. It's too innocent that you think she's lying. Jack: "He bought that?" Angelica: "I sold that." But he still hasn't actually seen the captain out on deck yet. So Jack calls a super top secret meeting with all the other men to discuss this supposedly ruthless captain of theirs. In a deleted scene (which you should totally search up and watch on YouTube), it shows Jack spreading the news throughout the crew. They all wisper "tonight" to eachother.
Eventually, a crew member goes to Jack and wispers "tonight", which is ironic because it was Jack who started it all in the first place! Jack: "Onto it then! Blackbeard. What are his habits?" Scrum: "Stays mostly to his cabin." Everyone mumbles their agreement. Jack: "Yes, but when he comes out . . ." Crew member: "He don't really come out." More concurring mumbling. Jack: "He has to come out sometime?" The same response from everyone: he doesn't come out. Jack: "Any of you sail with him before?" No's echo throughout the group. Jack is getting pretty impatient: "Any of you seen him before." Crew: "Not really, no." Jack: "Stays to his cabin, no one sailed with him, no one's seen him . . . Good news, gentlemen! This is not Blackbeard's ship. This is no the Queen Anne's Revenge." Now, does Jack really bleive what he is saying? Does he actually think that the whole Blackbeard thing is just Angelica's hoax? Scrum: "No, this be the Queen Anne's Revenge, right enough." Jack: "How do you know?"
Scrum: "I've seen the name, on the back of the ship." Scrum is not the smartest of Jack's companions. He's rather gullible. Jack: "Gentlemen, sirs, fellow conscriptees. . . you have been monstrously deceived." Crew member: "We are decepted then?" Jack just gives him a disapointed look and rolls his eyes. Jack: "Yes." He then scares them with tales of the Fountain of Youth and gets them to rise up against Angelica and her zombie crew so that they can actually sail under Captian Jack Sparrow, which they were promised to in the beginning. Jack: "Unless, we take the ship." Scrum jumps right onto that. He stands up, staps his sword into the table, and screams: "WE TAKE THE SHIP, THEN. NOW!" Jack just looks at him like he's crazy. Scrum runs off.
Jack looks at everyon else, and then shoes them away: "Go on, then." They grabs swords, wake up the other crew members, and storm out of there. Although it was Jack's idea, it doesn't seem like he's to fast to get out there and fight. He does take down some zombies, but I don't think he ever doubted whether Blackbeard was on the ship or not. He planned to just use this mutiny as a way to find out: to actually get Blackbeard off his lazy but and discuss the Fountain with him. Clever, huh?