This is French gray hardening. The process is very similar to color case hardening, except the quench is done in a solution of potassium prussiate instead of straight water. The solution can be varied to get different color, from a soft silver plated look to almost pewter color.
It is no more durable than case hardening colors.
Some people cut blue back to a silver color with acid and refer to it as French gray. But, true French gray is a hardening process, not a surface finish.

Best,
Ted