Try putting the blocks with the stronger colors in the corners. I'd try the ones with the red on opposite corners--upper right and lower left with the purples in the other 2 corners. As to border--perhaps, as you say--a narrow black to let it float and then one of those lavenders that's already in the piece. Maybe just stitch piano keys in that last border or some other simple quilting design. I'd be careful of making a border too busy with all the different designs in your blocks. Don't want to take away from those. Very pretty!