I know it's meant to be based on Historic real pirates/Treasure Island as well. But besides Mary Reed and Bartholomew Roberts who we could probably see later. It has every other pirate and storyline in it from that game.
For Black Sails to become AC IV, Flint or any other captain in the show would have to sink ∞ ships extremely fast, while at the same time doing impossible parkour stunts. And also he would have to have a superhuman tolerance to injuries from falls from great heights into hey carts.
That's to be expected since both are set in the same time period and using real history along with their plot. That's like saying ACIII was too much like the Patriot because it had the same characters and the same story line.
Overall for me I got the biggest Black Flag vibe off of the first season and each season after that has lessened that vibe as Black Sails has put their own twist on the history of the pirates.
I thought all seasons were a lot like the game, other than the plot/storyline. Even then, there are quite a few things that happened in the TV plot which can be found as 'game mechanics' in the game.
I thought it was awesome both were extremely alike, except for the plots they were not the same. Having played the game myself I found a greater appreciation for the show. I would say both are 90% the same when it comes to environments, "game mechanics," props, people, culture, etc. It reminded me very much of when I played the game.
Agree it was so cool that both these came out around a similar time. I absolutely loved AC4 and it's the only reason i got into black sails. There are a lot of similarities between the two, hell even flint is pretty similar to Edward Kenway.