Crabber69, check out the Blue Crab Archives (of which this forum is a part).http://www.blue-crab.org
It's a pretty handy reference and covers just about anything that you would want to know about blue crabs. The following is from the Menu Page, under Crab FAQs.
Q: How do crabs find food in the dark?
A: Crabs do not rely on vision like you and I do, when looking for food. Crabs cannot see very well and usually only use their vision when their prey is very close. Crabs find their food in the dark (or in the light) by something called "chemoreception". This is kind of like our sense of smell. They have very sensitive chemoreceptors (or smell organs) on their antennae and in their mouthparts and distributed throughout their body. So, crabs can "smell" the chemicals that their prey put out in the water and follow these "smells" to find food.
This information obtained from OceanLink Answers to Anthropoda Questions