The striping is probably due to him being a partridge, but a blue Silver partridge. The silver in the neck should start coming in as he approaches puberty.
Do you have pictures of them from day 1? Wanna make sure I didn't miss any clues in the chick down of Pookie?
The striping makes me think possible blue Cuckoo.
Blue X Black = 50% Blue, & 50% Black. In your mixing of those you can get leakage in the hackles, Saddle areas. Black can hide alot of other colors too, like Gold, Silver, & Red for examples.
Here's a BBS inheritance chart.