i have to guess - but the "black" ammo is the same idea as their ZMAX - put a snazzy box on some proven AR friendly loads, and market the hell out of it - maybe even sell it a smiggen cheaper like the zmax stuff. havent seen any on the shelf to know yet.
I went to the Wheeling, WV Cabelas store yesterday to buy some 450BM ammo for my upcoming SC hog hunt. All they had in stock was the 450BM Hornady Black ammunition. I purchased a box and when I brought it home I noticed the ballistic stats are exactly the same as the Hornady Custom brown box ammo I had shot previously. All in all I'd say there's no difference but the fancy box with the Gadsden Flag snake on it.