This technique is called the Black Magic technique. Imagine trying to stamp the flowers onto Basic Black cardstock with just the ink colors and the stamp. The colors wouldn't show up, you might see a faint flower image but not a vibrant blue or pink. This technique allows you to get bright colors onto a black background. The look is somewhat like colored chalk on a blackboard. but it wont smear like chalk.
You start by choosing a solid image type stamp, not an outline stamp, for the best result. Stamp your image with White Craft ink onto the black cardstock.
You will need to let it dry. You can use your heat tool to dry the ink faster.
This card uses the Flourishing Phrases stamp set.
I made a quick video to show you step by step.
I hope that you will give this technique a try.