I got it out last month and took another look at it. I bordered each block with a small strip of black and made the border from the leftover background fabric.
And, talk about a pieced backing! I used up a whole lot of leftover muslin scraps (almost 50, actually) to make this backing, and I incorporated the blocks I couldn't fit onto the front.
I decided to have some fun quilting this one, so I experimented a little and did something different in each block.