I don't think we can use black as the sashing. There are too many squares with a stitched black background and that poses the same problem as using white. Plus, black is kind of bleh. A lot of our squares have blue and green in them so maybe a colour that will match both of those? I think a neutral kind of golden yellow would work. There's that kind of shade in almost all of them but it's subtle enough.
As for the borders...no idea!

When I made my first quilt (just recently), I spent HOURS picking out colours. The women at the quilt shop probably hated me.