Ichigo's hollow form has always been white. That one appears to be black, without any of the characteristic markings that ichigo always had.. The shape, posture, etc are just too different.
Aizen had been interested in Ichigo for a long time because he knows who his parents were, and having such a lineage (part human part shinigami) would make him a most interesting test case for hollowfication. However it wasn't something he could just test, as specific events had to occur to awaken Ichigo's powers on both sides of things. He was unique in that he had an innate ability to become a being that could perfectly combine hollow and shinigami aspects, which is the form he had achieved when facing aizen for the second time. As Aizen had remarked, he couldn't sense a single thing from Ichigo, meaning that his level was so far beyond him, that he couldn't even comprehend it.