I have soaked a rag quilt w/black flannel in salt 3 times over night & in cold. It's still fading. Any ideas?

I have washed this quilt in cold and soaked in one cup plain salt over night 3 times. The dark red was so vibrant and the black just continues to fade. I have not tried vinegar since I do not know the type of dye it is, will try that or am I not using enough salt. Have used salt for years without fail, until now.