My Cuckoo is that dark too. I think most are. They have more black than the Barred Rocks do.
I found Cuckoo Marans this morning at Atwoods in Arkadelphia.
They had 5 or 6 pullets, and I bought 2, so as of about 30 minutes ago, they still have a few if anyone is close by. They're not feather legged or I'd have bought them all.
I have the one Cuckoo already, so these will go into that run when they're older, and I'll try to find a good feather legged roo to put with them.
The Cuckoos don't lay nearly as dark an egg as most of the other Marans though, but they're pretty. I have feather legged Wheatens too, which lay a little darker egg, but not as dark as the Black Copper.