User blog comment:Xilinoc/Ch. -547 - The State of Affairs op. 1 - "Expectations"/@comment-7488132-20130724170914/@comment-11051098-20130729080515

Since Ichigo only had his Shinigami powers awakened at the time, I guess that "true blood" was about that. Grand Fisher saw Isshin, if he recognized the haori around his shoulder, he could have guessed that Isshin was a former captain and "full blood/true blood" Shinigami, which would make Ichigo a "true blood".