You are absolutely correct. Most darker fabrics bleed. To test it, but the fabric into a white tub or sink. If you see color, it is bleeding. Put in HOT water, vinegar and fabric. The trick is to let it cool down on it's own. I suspect that is what is happening when you leave it for your half hour to hour. After it cools, test it again to see if it is still bleeding. I had to do some dark greens for my granddaughter's quilt two or three times. It will save you heartache later. If it is STILL not setting, I was taught to use salt, although I have never had to resort to this.