I made an accidental discovery this week that may be helpful, so I thought I would share.
I have been shucking seeds and my hands and fingernails are stained, as I do not wait for the pods to dry before collecting seeds. I was cleaning a piece of silver jewelry by rubbing toothpaste all over it to remove the tarnish (the plain white kind of toothpaste), and voila my nails and fingers lost most of the dark stains.
I shucked more seeds a few days later and tried it again, and it really does help. Here is a picture of my hand after shucking and several washings and a shower.
The second picture is five minutes later after a quick scrub with toothpaste. Not perfect, but way better.
My kids get really grossed out when my nails look like zombie nails, so I am glad to have found a quick fix!