Back on topic...
I don't know what crab you're going for, but I've never seen blues in the general Mass area. At times they can come as far as the south shore of the cape, but generally stay south of here. It takes really warm water to get them up here, as they like temperatures over 60 degrees and usually flourish in temps between 70 and 80. Temps over 90 are lethal.
Now there are some other crabs that inhabit our waters.
1. Green crab (shore crab): alien species from across the pond, this crab has flourished and taken over our shores. They're everywhere. Females are the characteristic light to dark green and generally small while males are larger and their tops are brown and undersides a yellowish orange color. There is very little market for these, as they're generally too small. However, they can be eaten, and taste as good as any other crab. I've seen some very large males that were very tasty! You can easily catch 20 of these in 5 minutes.
2. Rock crab: Reddish/purplish color with a somewhat rounded/oval shell. Can be large. Excellent eating (related to dungeness). Live just about everywhere, but have been pushed out of their habitats by green crabs. VERY AGGRESSIVE-Will remove fingers if large enough!
3. Asian Grapsid Shore Crab: Very very small guys with big fat claws. Not even worth catching. Another alien species from Japan. They steal bait.
4. Lady crab:Purple/grey spots on a beige background. Spiny shell, last pair of legs are swimmers. They are found on sandy shores and bury themselves in the sand. Edible, but somewhat rare north of cape cod.
5. Jonah Crab: similar to rock crab and dungeness, rounded shell, reddish/orange color. Telltale fat claws. Excellent eating. Again, very very agressive and will take off a finger.
6: Spider crab: Ugly and worthless as food. We've all seen them.
Every once in a while you may come across king crab in deep cold waters. And I've heard of dungeness being found here too.
Hope that helps! Time to go get some lobsters!