If you decided to use a color binding, I would go with the aqua because that is the center block and equal distance on all sides. I'm not OCD or anything, but I think not having that balance would bother me in this instance. I also feel that a black or charcoal might detract from the blocks, however, a gray that matches your wall color would blend in.
Good luck in whatever you decide. Your blocks are striking and beautifully done!!