I like using a dark sashing. I like the way it showcases the blocks. But I often use a very dark color, such as a navy, a deep burgundy or brown or even a very dark green. I find that the very, very dark color gives it a bit more warmth but being very dark it doesn't actually show as color. But, as with so many things, lay your blocks on it and leave the room. Each time you enter, think about what you see. It is just ok or do you really like what you see.