Mottled and black are the same bird with the exception of mottled carrying two mottled genes.
Problem is they are worlds apart from mille fleur.
Black/mottled should be silver based. MF are gold based. MF should be partridge and B/M need to be extended black. MF also carry columbian genes as well.
You'll need to cross in the silver base and the extended black. At the same time breeding out the gold base, the partridge, the columbian genes and the mottled genes for the black.
It will take a ton of chicks and a few generations to get there.
With blacks and mottled already available imo it would be much wiser to secure them from somewhere then to try and make them.
Even if it takes a couple years to find them you'd still come out ahead time wise and a ton less work and throw away chicks.