The blues in New England waters are among the largest in the world, and we have plenty of them. Dusky, Carcharhinus obscurus - has a ridge on its back between dorsals. The dusky does not venture north of Cape Cod. Small duskys and sandbar sharks of the same size are very hard to tell apart. Both species are on the south side of Cape Cod.
Some people use a magnifying glass to separate the two species.
They look at the scales. If they don't overlap it's a sandbar. But they are not seen in coastal New England. The IGFA record for a dusky is 764 lbs. In 1999 the Dusky became a protected species.