I copied the list from McMurray's web site. It does list Dark Cornish -- could they be dying at 8 weeks old like Cornish X?
"Each of these assortments will give you at least 5 different heavy breeds. Our choice of varieties, but we guarantee at least 5 different ones from this list: Black Australorps; Lt. Brahmas; Dark Cornish; Black and White Giants; Buff and White Orpingtons; New Hampshire, Rhode Island Reds, Barred, White, Partridge, Buff Rocks; Sussex, Turkens; White, Silver Laced, & Columbian Wyandottes, Red Star and Black Star." - McMurray Hatchery website