Prismic's Image optimization does currently not support blurred placeholders. imgix however does.
I was however now advised that there is another ticket that refers to exactly the same and it seems to be on the Prismic's roadmap to integrate: Support for imgix blurhash. I guess it will not be for next/image, but for Prismic/ImgIx own solution.
I would however be interested how prismic's solution compares to next/image in terms of performance for server side rendered pages as well as client side rendered ones. Has anyone looked into this?