If you are gathering, you can chase them by hand in the shallows or haul trawl from a boat. By hand is more fun and a LOT less work. The object of your desires is the muscle holding the shells together. Cut it close to both sides and saute' gently in a manner of your choosing. Crabs are everywhere.
Blues are the target of most crab hunters.
Blues are most often found along the edges of salt marshes and muddy flats and shores. They are easiest to harvest at night. Bring a bucket, net and a flashlight. Wade the shallows where earlier you swam and try to let them swim into the net. Their claws cut like kitchen shears.