that just about explains my experiences. Short and sweet and easy to understand.
Walnuts ripen in the fall. As the fruit matures, the hull softens and changes from a solid green to a yellowish color. The fruit are mature and ready for harvest as soon as the hull can be dented with your thumb. The best quality nuts are obtained by picking or shaking the mature nuts from the tree. Most individuals, however, gather the mature nuts as they drop to the ground. Dropping of mature nuts usually occurs in mid to late September. Before you spend a lot of time gathering nuts, it's a good idea to crack a few to make sure the kernels are full. Nuts occasionally fail to fill or have small, shrunken kernels. Nut crops vary from year to year. A tree that produced bushels last year may have many or few nuts this year.
The nuts should be hulled immediately after they have been harvested. If the hulls are allowed to remain on for any length of time, the juice in the hull will discolor the nut meats and make them strong tasting. The stain also discolors skin, clothing, concrete, and anything else that it touches. There are various ways and devices to hull walnuts -- a cement mixer, corn sheller, automobile wheel, and squirrel cage are possibilities. Hulls can also be removed by stomping the nuts under foot or pounding with a hammer. After hulling, thoroughly wash the nuts to remove hull debris and juices. Small quantities can be washed in a large bucket or tub. At this time, the good nuts can be sorted from the bad ones. Unfilled nuts float while filled nuts sink. (Rubber gloves should be worn when hulling and cleaning to prevent staining of the hands.)
http://www.ipm.iastate.edu/ipm/hortnews/1994/9-16-1994/bnut.html
It also had some things I didn't know, like that you can sort them by putting them in water. I remember that my folks always used a hammer.
Since your hands are stained so badly, I think you have your answer about whether you have black walnuts. Use plastic gloves, and I bet you still have a mess and stained hands. But since the article says that they usually ripen in September, I wonder if the ones falling from yours are just not ripe yet.