With all those dark and light fabrics, you may need to use medium color to be able to see it on both dark and light. Or at least a brighter color that won't blend into both colors. At it stands now, the dark thread is invisible on your printed blocks. Odd as it seems, I'm thinking maybe a chartreuse green or something in that category.
I think that the variegated thread would also tend to blend into your printed blocks, and not really bring out the full pattern on the lighter color. That is my humble opinion.
By the way, I just LOVE your block and the colors. Your piecing is meticulous! Beautiful job.