No need to continue the 3D effect. I think you will take away from the center if you try to replicate 3D in the border. A single thin border of blue all the way around that you mentioned and then one or both of your grays on either side will frame this beautiful AW quilt quite nicely I think. You could make like piano key of alternating light and dark gray running in horizontal "coin" stacks on either side or go with just a solid.