Before I had a design wall, I would lay out a sheet or large piece of fabric on the bed. I would arrange the blocks and leave them. Whenever I would enter the room, I would look at it and usually something would pop out as not right. I would move them around and leave. If it took more than a day to figure out the right placement, I could roll up the sheet and keep them in the correct placement. But, usually, it worked itself out quickly. I think the trick was leaving the room. This gave me fresh eyes to look at it. Same trick works for sashing color or borders. Put the different colors on opposite sides and see which side catches your eye and makes you smile.