I would go with a medium gray, that will blend in the gray and purple blocks and soften the black.
Personally I'm not a huge fan of how variegated thread blends at some points and stands out at others against the same fabric. It makes the quilting lines look "broken" to me. Of course, a lot of the time it looks great, so if you like it go for it... Try puddling some of the thread on different blocks first to see how it looks.
PS: just had to add, I think your quilt looks great! Very modern, and I generally avoid black, but I think it works really well here.
