OK, wait.
If you wanted to put in sashing to make it bigger then here's an option:
The only sashing I would add would be from the dark fabric with another turquoise square in square in the middle (turq on point with the print corners) - the same as the strips you'be put around the block. The cornerstones should be dark as well - or incorporate them into the sashing strips. But they should just blend in.
So now there would be a chain of 5 turq's diamonds connecting in each direction.
That would also mask the fact of the sashing because it would be such a integral part of the pattern.
You couldn't tell the difference between the block and sashing components.
Does that make any sense?