A dark pied bird is a pied bird (2 copies of gene) that displays no white (or just white on the throat and flight feathers). It's different from a split pied bird, which is a bird with only 1 copy of the gene (ie, only 1 parent was pied).
Personally I'd like to know more about the pied genes in general. I was recently informed that in pied horses, the genes display in 1 of three patterns. In ball pythons, you have three "expressions" of pattern (high white, mid white, and low white expressions). If this is the case across pied genes/species, there may be some pattern in pied peafowl that we just haven't bothered to document yet, including Dark Pied simply being an alternate expression of pied, giving us the same "three expression types" as ball pythons- loud pied (high white), pied (mid white), and dark pied (low white).
In addition, I was originally told that there's no predicting the way the white will display on the feathers (implying that the pattern is completely random). However, I have made a habit of purchasing pied hens with a white "saddle" over their wings, and outside of the one loud pied and the dark pied babies they have thrown, the same white saddle coloration has shown up in all their pied offspring, suggesting the particular white pattern they have may be hereditary.
So, I think we have a lot to explore and learn when it comes to pied genetics.