I thought the bottom row was all blue and white with the last 3 very dark blue navy. But because the focus fabric has blue, black, and white besides the purple I think blue and white as well as black and white work. And looking even closer the print second from the left on the bottom looks like it is a complimentary print of the focus fabric.
If you do go with a sampler all of these could work, maybe even the reddish purple if you use it with the batik 4th from the right and the other reddish purple and then the ones from the bottom row. And maybe get a solid or tone on tone or a color that seams nuetral for the sashing between the blocks. Maybe wait until the blocks are finished to pick that out.