Is this a Marian?

[NatJ]

I bought her in San Joaquin county California. Her eggs are medium to dark brown. About medium size eggs.

Assuming you bought her as a chick from a store (not hatched from someone's backyard flock), she certainly might be a Marans. Or she might be some mix that includes Marans and some other breed. The egg color and the foot feathers certainly suggest that.

She is definitely not a Cochin.

I don't see why it couldn't be a blue birchen Marans. It's unlikely, but possible. Meyer hatchery sells them. MyPetChicken sells them. If it is a Marans it should lay dark brown eggs.

That sounds reasonable to me.