It really looks like a sail, but there a couple of resons I STILL say it is a blue. First - Body shape leans toward a sail, BUT, when the fish is upside down there is absolutely no evidence of a sail. Also, there are pictures of blues exactly like the original picture posted. I know sails can put their sails down, but there seems to be a distinct end to the dorsal fin. I can't blame people for thinking that is a sail, but it will be tough to pursuade me. Second - I know Stephen and he had the fish boatside but in the midst of the chaos he was unable to get a perfect photo, but if he said it wasn't a sail, it wasn't a sail.
Look at these pictures -
http://www.swordfishingcentral.com/g...raphy/IMG_8901 - notice the flap of sail coming down
http://www.swordfishingcentral.com/g.../blue_marlin_l - definitely a blue marlin with the same exact shape, just a little thicker