I agree with black - but be careful, as black can be very difficult to match! Can you contact the shop that ran the BOM to see what black they used? It will almost certainly be a different lot, but at least you could then use the same manufacturer. If that doesn't work, I'd use a sashing to avoid the blacks of the blocks and the blacks from the alternating squares. The one Betty shows above is gorgeous! It's a pretty thin sashing strip. Using a batik that has some of the colors of your blocks would work. Alternatively, I'd go with a gray or a color that you want to pick up from the blocks. If you go with a cream, be careful as you don't want the block on the bottom right to get lost.