OMG!!! Do not apply black walnut juice directly from the hull to any part of your skin undiluted!! It will cause a nasty burn and stain more like a wound that will last for up to 3 of 4 weeks.
While making a tincture I splashed a few drops on my arm and hand and it looked like a chemical burn which took some time to heal.
The black walnuts I use are the species native to southern California and may be stronger than the common English black walnut I'm not sure however, I do not recommend applying the juice from the black walnut as it also causes deep staining when the active substance Juglone is oxidized from air exposure. It will also stain your fingernails for weeks!