its probably Dun(I^D) or the Dark Brown columbian Restrictor Db(will give ER birds a Dark chocolate color)Hm... do you have pictures of this chick?
The Choc gene is a mutation and is very rare, it would be something very special for it to just pop up in a Marans line.
I'm very interested and would love to learn more about this flock's history.
I believe this chick to carry Db or Dun. as Chocolate is none existant in Nicaragua