I used grey thread in my Spiderwebb quilt because of variety of scraps however the centers were black. I ended up using a Sharpie to colour the threads in all those spots. It wasn't really noticeable until the quilting kind of separated the seams. I know it will relax when washed but I carefully coloured the threads until I wash it because it really bugged me. Try a black Sharpie on a couple of scraps sewn together and see if it will work for you.