Beadiste, I agree, the blue represents, I'm almost sure, individual leaves. BWDIK? :)

Pics below are a "Kenzan" jar (linked) and a close-up of the blue area on the bead. You can see leaf margins and midribs have the same bold black outline, with some terminal leaf tips unfinished/implied. The leaf coloration is also not uniform, but appears "variegated" with multiple brush stokes.

https://www.liveauctioneers.com/item/27768119_a-japanese-polychrome-pottery-kenzan-jar