The parents of the bird are normal according to the breeder, the siblings are normal (the normal bird on top are half-sibling), one have a bit more black on the belly but isn't nothing compared to this.
Overall they are bit darker then the ones I have but it's because mine have contact with direct sunlight and the breeder that sold me this birds doesn't have, or at least not enough sunlight. But they all fit in the variations of plumage of the (sub)species, except the belly of the "melanistic".
The beak isn't silver, was the light.
If isn't a true melanistic bird, possible is some kind of mutation.