My question is, should recessive white give you pure white BCM chicks or white with black flecks like mine?
Recessive white should give you pure white chicks.
White with black flecks is usually from Dominant White, which turns black to white but can be a bit leaky. If your hen produced chicks with Dominant White, then their father would have to have the Dominant White gene, which would cause white in his feathers too (which would mean the hen mated with another rooster that you didn't know about.) Or if the eggs got mixed up, and you were actually hatching ones laid by a different hen (could be a hen with Dominant White.)
I had wondered if you had two Marans with blue, because you might have chicks with splash. But when I look at the pictures, I do not think that happened.
A third explanation would be if each parent was carrying recessive genes for another color pattern, and some chicks are expressing that pattern (example: Silver Wheaten could cause something like what you first described, but the pictures do not look quite right for that explanation either.)
the chucky cockerel below looks like he is silver with baring on his head.
View attachment 4283126
I do not think that cockerel has the barring gene. I think he's got some interaction of other genes causing that appearance (similar to how Egyptian Fayoumis look sort-of barred but do not have the barring gene. They have several other genes interacting to make their pattern.)
If that cockerel came from the black hen and the black & copper rooster in your pictures, then the hen must have the silver gene. That would mean she is not a Black COPPER Marans, at which point I would wonder what else is going on genetically. If the black hen has the silver gene, she would give the silver gene to her sons and not to her daughters. (If you also have pullets that show silver, this would mean I'm wrong about the hen being the only source of silver for this batch of chicks.)
Some hatcheries have been selling Marans-mixes this past few years. Maybe your hen is one of those? That would be an obvious explanation for some unexpected colors of chicks.