User blog comment:Xilinoc/Ch. 679 - THE END/@comment-28850184-20160626054720/@comment-5398287-20160627004901

Those veins have shown up a couple of other times, actually. Back when Yhwach was fighting Ichibei, his attempt to destroy Ichibei's body with Blut Vene Anhaben involved the veins covering the left side of Ichibei's body, and later on when he took control of Ichigo for a few seconds using his Quincy blood/Reiatsu, the same veins appeared all over Ichigo's arm to represent it being controlled. So in Yhwach's case, the veins can also appear when he's negatively affecting his opponent in some capacity.