If you want to see how a narrow dark sashing would look, you can check out my quasi-crazy quilt posting from last summer:
http://www.quiltingboard.com/picture...y-t222232.html
It is made with QAYG attachment of blocks, resulting in a 1/2" finished sash. I like the idea of black sashing and considered using it, but my eyes have trouble with black. The dark purple was a little easier for me.
Your decorative stitching is beautiful -- I wish I had that capability on my vintage machines.