Glad to help and what an enabling partner you have, lol And congrats on your marriage!
I've had black skinned NN for a couple decades. Got the first ones via an egg trade, was supposed to be NN for NN but I was surprised at the black skinned ones. Tried to get more information but combination of language barrier/poor contact plus they just disappeared.. so no idea on their history. The originals came in a variety of colors(partridge, red, later breedings threw whites) but the black skin was best on the blacks so I just bred them in that direction. They basically look like NN cemani.
There are others on here with black skin NN.. also black skinned NN by the way of crosses with silkies are somewhat common. And then there are the Showgirls- NN silkies.
As for your source of fibro, have you talked with others about differences between cemani vs svart hona. I haven't followed threads on those much, noticed one tidbit that might be of interest to you- svarts allegedly are a little more dual purpose than cemani?
Can't figure out FLF?