I'd vote for navy or black, but dark brown could look nice, too.

Your blocks are GORGEOUS!! Wow, real eye candy. What about grey thread?

I might look to see if you just put the blocks together and let the centers 'float', how that appealed to you. Otherwise I would probably go for black for a sashing. Beautiful blocks, btw.

I would do a variety of auditioning and see what you like best. You could "float" them with using your background fabric or try a variety of sashing such as grey, black or dark brown. Keep trying until you find what you think is pleasing. Very nice blocks !!