There's alot going on already with the colors and pattern and a plain dark blue border would really keep the focus on the top of the quilt and keep it from looking to "busy".
What I do is put the quilt on the bed and leave it there for a couple days. Every time I walk by the room I look at it to see how it looks, as it is, and it gives me an idea of what I think I should use for a border. Sometimes looking at it from a distance helps. Atleast it does for me.