On certain breeds, the shaved hair can grow back a different colour/texture. I have seen this on Siamese cats after surgery & iv fluids. So yes, the client probably meant her cats fur grew back darker than the surrounding fur. This is why we spey Siamese type cats midline, but most cats we spey from the flank.
We have one client who we performed spinal surgery to remove part of a tumour. He was put on chemotherapy & steroid afterwards, and his hair has NEVER grown back right along his spine!