I like the first blue you have or the last. The taupe is blah and blends too much. The dark blue too stark, not soft. the second blue goes well, stands out a bit but not too much, you have very few blocks with enough blue on the edges that they would disappear with blue sashing. There might be a better or more elaborate choice out there but the blue looks great. I also wouldn't get too fancy with extra borders -- the blocks are the main attraction here.