I think your idea of using black as a binding is an excellent one. The quilt is so adorable, the black binding would keep your focus on the cute cars. I use 2 1/2" strips WOF for binding, which I fold in half lengthwise and press. Then I do a 1/4" seam to attach the binding onto the front of the quilt, and fold over the binding to the back. That way you have a nice finished folded edge to blind stitch to the stitched seam on the back. I use this for all my quilts, and find it's a lovely neat finish.