I have not read all the replies, but agree with most of them that suggest redoing the black border. Something else you might want to consider is after the black is of, steaming, no pressing or ironing but steaming the bias of the triangles may help to reshape them. Another suggestion is to cut your black fabric on the lengthwise grain of the fabric, this has practically no stretch. One more suggestion a variegated thread rather than a solid.
Beautiful, Beautiful quilt and border, I am sure what ever you decide to do, it will be Outstanding.