I think you could get them to breed true, but it may take several generations. I would search Fibromelanistic Easter Egger in the BYC threads and see if others have described their work. A simple google search may also give you some useful information, granted you would have to sift through a...
You are correct fibro is short for Fibromelanistic which means the birds have a large amount of melanin in their skin and bones. Examples of this are Silkies and Ayam Cemanis as well as the Swedish chicken, Svart Hona.😊
I do not have experience breeding Ayam cemanis, but do breed silkies, another fibro breed. You can breed your Cemani rooster to lavender ameraucanas, but the chicks will all be black and carry the lavender gene. If you cross a cockerel of this crossing back to its lavender ameraucana mother (or...