I would do something 'swirly' in the centers to contrast with the linear blocks. If you feel it needs more than that, perhaps a curvy line in the middle of the outer square. I might do it a bit off center - closer to the inner square. I would use either a variegated blue thread or use 2 thread colors, a dark on the light blue sections and a dark blue on the lighter sections. Please post when done so we can see what you decided to do.