To my knowledge, a bitch stripe is pretty common in WGSLs. My boy is a WGSL/West German Working Line combo (also a long coat, almost 9 months old) and he's got a definitive bitch stripe down his back. Nothing wrong with it in my opinion, especially since the rest of the black pigment on my boy is a really nice black with no fading otherwise.
Your boy is handsome! It does look like a bitch stripe but that's not a cause for concern whatsoever!
Rocket (pomeranian) - 01/2002 - 03/2016
Hircine vom HausReid "Sawyer" (GSD) - 11/24/16