What would cause what? The darkening of the skin? Lack of fur?
I'm not exactly sure what you're looking at. That's why I ask.
My male GSD had something that looked similar. The vet removed the actual testicles from the scrotum, and the scrotum remained, kind of like a bag of floppy black skin. It took a few months to shrink and now it looks like you would expect a neutered dog to look...just a slight furry bump between his legs.
Does that help?
Or are you seeing something else? If so, please explain it and we'll try and figure it out.
A few months...at least, that's what I remember. After a while, I just kind of stopped noticing.
Maybe someone who's had their dog neutered more recently will recall exactly. I had Camper neutered at 14 months. He's 22 months now, and he's been fully shrunken and fur grown-in for ... let's see... at least a few months now...