If you decide to do the black with the 4 patch cornerstone, I would rotate the blocks and the 4 patches, so that the color in the block was closest to the same color in the cornerstone (if that makes sense). In your picture the blocks on the right are the the way I would place the color in the cornerstone, where on the left they aren't. I think that would give the quilt some symmetry.
This is one of the reasons I love my EQ... I can recreate the blocks in the program, and play around with different ideas on what I want for sashing, borders etc until I find something I like.