you can use bleach..
fold up a few paper towels or rag and put them/it in a shallow pan ( take out plastic food covers work nicely), choose a stamp , place on the bleached towel, try to ensure you get full coverage, stamp on black CS. you can use a heat gun but its better if you let it dry overnight. after CS is dry use a watercolor wash or acrylic paint wash or anything else you think of and go over just the area where you bleach stamped. That area will hold the color & really pop from the contrast. Note:you won't/shouldn't see the watercolor wash spill over onto the black, but if you use an acrylic wash you will get some spill over onto the Black, which can be quite interesting too....