We had several from the two homo black bulls we got from Select Sires, I actually contacted them about it.
This was their reply (after much discussion):
When Charolais is mated to black, as long as Charolais 'diluter' gene is present, you will end up with calves that are some shade of gray (may range from dark gray to nearly white)
Yes, the odds seem unusual in the case of your cows/calves, but it's possible
So that diluter gene must either be present or not... I'm guessing the % of char doesn't matter... just if they have that gene in them at all.