A true shadow box quilt will not have the "shadow" fabric going the full length and width of the focus fabric. The shadow fabric needs to have a square of background fabric sewn on the end to give the 3d effect. I can't tell by the photos but if you sew the black strips you have to the opposite side of each block will they be squares or still rectangles. If they become squares, then I suspect the pattern the original piecer had in mind was this one that barnbum made a while back:
http://www.quiltingboard.com/picture...d-t135766.html
She said she got the original pattern out of Quilt Magazine back in 2011