I just flipped through 2 of Eileen Wright's bargello books to see what she used. Some had no borders but others had a narrow border of brown or green or picked up the color she wanted to highlight in the quilt--such as purple. In your case I'd preview a bit of that dark blue in the upper right corner or what about a deep brown-red that's in the center but even darker? Personally, I like a border on the quilts I make as it frames the work and gives the eye a place to stop.