Oh... I thought you meant the lighter shade. The pullets were in the forefront, so I assumed you were referring to them, and the cockerel in the first pic looked pretty typical.
I don’t know if the parents are hatchery or exhibition birds, so I’ll address both, though that porcelain looks too nice to be a hatchery bird. On hatchery birds, anything goes, so a cockerel might have a pretty dark neck, and faults in exhibition Porcelains are harder to detect because the colors are diluted so much. I wouldn’t be surprised if many porcelains were almost solid black on the neck beneath the mottling and dilution, because you just can’t tell.
Not so much a recessive gene or mutation as a natural variation.