Black - the second anal fin on a blue is forward of the second dorsal. From the pic it looks like the second dorsal is forward of the second anal fin meaning it's a black.

From another website. An interesting read.

"My way of working out if it is a blue, black or stripy is simple.

First thing I look at is the tail. Stripys have a very straight flat looking tail almost v like in shape. Blues and blacks are more curved and look like a flattened u shape with the tips slightly curled in at the ends sometimes (the tails look powerful compared to the smaller stripy).

The next thing is dorsal fin. On a stripy it is huge the whole depth of the body. At times they can be a little smaller but they are nearly always around that size. A blue is around half the body depth and black can be half that again. Having a curve near the top of the dorsal makes no difference to fish species.

Body is the next thing to look at. Stripy has a very long slender looking body even when they get up in size. You put a 160kg stripy next to a 160kg blue and you will notice a difference straight away. Blues are solid fish and are built this way because they move fast. A black normally has a huge head compared to a blue but the rest of the body shape can be very much like a blue.

Bill is the next thing I look at. Stripys have very long narrow bill. A blue has a solid long bill very much like a stripy also. Blacks have short almost round baseball bat type of bill. Smaller blacks not so much round but are still short. When they hit 400lb or more it is very easy to see.

Anal fin is the next. Blues have huge anal fins can be almost half the depth of the body in that body region. Blacks have smaller anal fins. Almost half to three quarters that size again. Stripys, well if you can’t tell if it is a stripy by now I’d send a flesh sample away to get id.

Pectoral fins are the giveaway between blacks and blues. Just the shape is enough to tell. Blues are straight and narrow and can be curved on the tips. Blacks are curved and pointed at the tips. Also blacks are fixed rigid and will not fold down if they do you will break them to do so. Blues may fold down on most fish but there is the odd fish that doesn’t.

Last of all is scales, stripy has them under the skin. Blues are on top. A lot of times while moving fish into or around the cockpit they will come off if it’s a blue. Scales too can be very hard to work out as from information I have found blues can have 1,2 or even three points to their scales and blacks and stripy have the same looking scales except blacks are bigger. The shapes can vary depending on where on the body they come from I have read also. i must say i have never had to use scales to id a marlin yet.

All these fish also fight different when hooked stripys do a lot of jumping and greyhounding. They also don’t pull string like a blue does.

With a blue, most of the time I have found it goes north once hooked. Don’t ask me why but nine times out of ten it will head in that general direction. And when jumping, it is fast and furious. They often do this half body out of the water grey hounding in circles. They just tear the surface to bits.

Blacks tend to do more leaping out of the water straight up then down. Almost like a leap and a plop. They also do a lot of kicking from side to side while in the air. The head looks like it is thrashing from side to side when it leaps."

Thanks Alan

I was just going to say the blacks pectoral fins don't fold down! Plenty more to it, thanks Alan.