I quilted one with a bright front and a dark batik backing and changed colors with each section, using matching thread top and bottom. The result was a very cool-looking design on the back.
I personally wouldn’t trust just a dark bobbin thread, since it might make dark dots in places on the top where you don’t want to see them.
Why not do the main part of the fire in some “fiery” shade, both top and bottom, and then quilt the background and sparks in something dark?
hugs, charlotte