I'll let you tell the French who keep Marans this.
It is true the the standard for what it's worth accepts various shades. However, if you cross the border into France and speak to the locals there is a type of snobbery, for want of a better description about what is, and what isn't a Marans. They don't really care what someone says is allowed for shows etc. To them, the Marans is black with feathered legs.
It's much like the Catalana del Pratt and the Black Minorcans closer to where I live.
There is some pride taken in the local breeds which often have qualities that people who set breed standards don't accept.
In general it's not to be taken too seriously.