Rachael, I also think the brown border extensions distract from the beautiful center of the quilt. Although it's a little work, it will be well worth the time spent to take it out and put a black piece there. If you decided to do a design in the black border, those extensions may get in the way of it. OR, you could do the same brown extensions on the other black border so it looks like you are framing a black square in each corner.
This site is the place to get opinions, and that's my two cents! :D
By the way, the quilt is absolutely gorgeous!! :thumbup: