Blue Starch

I found some blue starch granules on the stone slabs from the archaeological sites in Japan.

I haven't seen these kinds of starch graules from modern reference samples. Why these starch granules have blue colour? This is one of research questions.









Photo 1: Blue starch on a stone slab from the Sannai-Maruyama site in Aomori (400x)











Photo 2: Blue starch on a stone slab from the Tsukuda site in Hyogo (400x)