Question: why do dolphins have blowholes and is it possible for a crab to have one

The blowhole is the nostril of the dolphin. Dolphins are mammals (like us) and breathe air. We breathe through our noses, they use their blowholes.
Crabs are not mammals and do not have a blowhole. Instead they have gills (like fish) but unlike a fish which cannot breathe out of water, crab gills can still function id the crab is on land.

You can find out more about crabs at: http://thebluecrabresources.weebly.com/respiratory-system.html