Do you have any of the lighter, center square fabric left?
I wonder how that would look to separate the blocks yet keep continuity.
Will you be working black into the fabric in the surrounding borders (maybe a print with black?) or binding (the same black used for sashing?) or even the thread for quilting? Black would work then...but I wouldn't go too wide in the sashings.
The black sashing looks good next to the blocks that have some black in them. When you work up the FINAL layout of blocks, try to make sure to spread those blocks with black throughout the quilt. It really helped me to take pictures in *black and white* to see the true value in the colors of several quilts I've made in the past.
Got enough black to lay out a corner section? Let's see :)