Blue Crabs are used for bait for many inshore and near shore species of fish. Inshore blue crabs are used for Drum. Off shore or near shore they are used for just about anything. I have had success using them for Cobia, Grouper, Sharks, Barracuda and Tarpon.
Blue crabbing is a simple thing to do for the commoner. Simply purchase a triangle trap that goes for Nine dollars or so at any sporting goods store that sells fishing supplies.
To use this type of trap all you will need is chicken necks or legs. Simply bait your trap and lower over a bridge or pier until it reaches the bottom and opens up flat wait ten or so min’s and raise your trap. The trap sides will close in to complete a triangle capturing you a crab. Have a bucket ready to hold your crabs. Please see Blue Crab regulations for local restrictions.
The other more convienent way is to purchase a cage type trap. This trap will run around 25 to 30 dollars . Simply bait this trap lower to the bottom and leave it alone for the night. Check this type of trap the next day or when you are finished fishing. When using a boat with this type of trap keep in mind the local regulations. Also do not lay your trap next to or around any commercial traps in the area. The commercial trappers do not take kindly of private traps in their area and sometimes you will find your trap mangled and or completely missing all together.

A more exciting way to kill some time while hanging out on the sand bar is to grab your fish net and simply scoop the crabs up while they swim by in the current. I have done this on the SandBar North of Caladesi island in 2 to 3 foot of water. On a good day you can get 10 to 12 blue crabs in about an hr.

If crabbing for eating purpuses here is a link for cleaning your catch there is some work to cleaning blue crabs but the meat is very sweet.
Make sure you have a bucket for your catch to contain them, the last thing you want is four or five large angry crabs crawling and snapping around your deck while your in your bare feet.