If i pick them green to keep them producing can i air dry them for long storage

I don't know. I usually eat them fresh. I also like them in the pod like a string bean.

When I pick them how can I tell whether to use them as snap beans or whether to shell them?

I pick them a little greener for the string bean application. Try it and see if you like it. I do it with other southern peas. I can't remember if I did it specifically with Black eyed peas. Usually I would grow a birch of different types. They are really good soil improvers. I would just turn the vines into the soil.

In the ones you want to use for shells, let the pods fill out with lumps (peas) fully formed inside. The ones you use for snap peas should be about 6 to 8 inches long max. with smooth sides. I usually allow the peas to fully form and use those for shell and pick the others at the same time that are about half grown and use those for snaps. Mix about 90% shells and 10%snaps. They freeze well, no need to blanch. Just put in a ziplock bag and freeze.

If using for shells, they should zip on one side and should come out without a milky film. It the film is still present and the peas do not come out easily, you picked a little too early.

If using them for snap beans you might be able to get a crop for harvest. It would be cutting it close. If they do not make it they will still help improve the soil as long as they are inoculated.

I like purple hull peas better than blackeye peas. They have a better flavor and are pretty productive as well.