My artist's seal (called "rakkan" in Japanese) reads "Sokyu", which means deep blue high in the sky.

Rakkan seal, artist signature on sumi-e
Origin of Seal

seal, in an East and Southeast Asian context, is a general name for printing stamps and impressions thereof which are used in lieu of signatures in personal documents, office paperwork, contracts, art, or any item requiring acknowledgement or authorship. On documents they were usually used to print an impression using a pigmented paste or ink, unlike the wax impression commonly used in Europe.