OK, here's hoping I've had enough coffee to be coherent ...
I have been reading on the Fm gene that gives the Silkies their black skin, meat, and bones, and it can/will be diluted when crossed to a non-Fm breed, similar to the NN gene being diluted in Nn form. My customer tells me that when they find Silkies in the market, they are smaller than the ones he gets from me. I am supposing this is a function of age, as I wait until I am certain a bird is a cockerel before I sell to him, so they tend to be older and more flavorful (which he likes very much). That is why I am happy to find a somewhat local source for Swedish Blacks. The person posted a pic of both a Svart Hona and a Cemani (both cockerels) next to each other,

and it was easy to see which had more body.
Go back a couple pages and look for the link to the ALBC's pdf on selecting for meat qualities, and that will help you choose breeding stock for more than just weight. Silkies/black Fm birds are used in soup in traditional Chinese medicine, so older birds are just as desirable as the young ones. Talk to your future customers, and you may find that they will not want diluted gene birds.
I got the Silkies because I want broodies - we have about a nine month storm season here where power can get knocked out, and we are the "end of the line" at the dead end of a dirt road. Cull Silkie hens will be put to work hatching eggs and brooding chicks. My customer not only will buy every cull cockerel from me, he says when I get really rolling on producing them, he'll happily promote them to others he knows who would want to buy.