Thank you Whateve for your reply!Getting much darker from conditioning is a sign that the surface has worn away, similar to the way scratches look darker when conditioner is added. There isn't much you can do to fix it. You could try a thin coat of leather cement or leather refinisher.
It totally makes sense to me why the surface of the strap turned dark. I'm all right as long as it's not rottening. I don't have experience with leather cement or refinisher but maybe someday down the road i'll check them Out.