Sashing is the strip of fabric that you would place between the blocks. It can be one inch or two inches wide, and the same height of the block. I would keep the sashing the same width as each of the strips within your blocks........ just enough sashing to make each individual block stand out. I think if your sashing is too wide, your eye would focus on the sashing more (since it's dark) and not so much on the blocks. You want your eyes to focus on the blocks.