User blog comment:Xilinoc/Ch. 579 - THE UNDEAD 6/@comment-29183117-20140507174805/@comment-5693502-20140507233203

Despite Ulquiorra mentioning that both Cero variations were used by Espada, the Gran Rey Cero holds more credence as an Espada wide technique (at least in my opinion) simply by the fact that he said it was one of two things forbidden within Las Noches. Also the Cero Oscuras just looked like his reiatsu in Cero form, given an appropriate name; to me, all I see there is a normal Cero given an upgrade (in appearance and power) due to Ulquiorra being in Resurreccion (in a similar principle to Ichigo's Getsuga Tensho being coloured white in shikai and black in bankai).