Finding Blackbeard's Treasure in Black Flag Resynced is actually a lot tougher than the game's other treasure maps, mainly because it doesn't give you any coordinates, forcing you to rely on your knowledge of the Caribbean. This is a new quest that was added in Resynced, but you won't get it until after you complete the Do Not Go Gently main quest.
Edward finds the treasure map hidden in Blackbeard's hat, which starts the hunt, so you can't miss it. Below I'll explain where the map points to, and how to navigate the area it's contained within.
Where to find Blackbeard's Treasure in Black Flag Resynced
The location Blackbeard's Map points to is Anotto Bay (coordinates 590,245) to the north of Kingston on the north coast of the island, in the Charlotte region. When you arrive, you'll find a bunch of pirate ships also laying claim to the treasure, who you'll have to sink before landing the Jackdaw.
Finding the treasure is relatively straightforward. From where you land, hop across the boats to your left and dive down, looking for the sunken mineshaft. You'll have to navigate these tunnels, dodging sharks and sea anemones until you reach an area with wooden platforms, gunpowder barrels, and a bunch of enemies. Pick up one of the kegs and take it over to the far side of the room where the rubble is (this will glow green when you hold LT on controller).
Place the barrel down and shoot it to blow up the rubble. Continue on until you reach the cave where you have to confront Blackbeard's former sailing master, and then grab another gunpowder barrel and carry it up the path to the right of the waterfall, where you'll find a second pile of rubble to destroy.
Inside, you'll find the treasure, though you'll have to head back to Great Inagua afterwards to actually complete the quest. As a reward, you'll get Blackbeard's portrait, which you can hang in the loot cave area of the manor.
