Line a baking sheet with parchment paper or plastic wrap.
Melt black candy melts in a microwave-safe bowl according to package directions. Repeat with blue and purple candy melts. Add blue and purple food coloring to them to make the colors darker, if you'd like.
Transfer blue candy melts to a Ziploc bag with a small corner cut off. Repeat with the purple candy melts.
Pour melted black candy melts onto the prepared baking sheet and spread into a rectangle about 1/4-inch thick. Drizzle with purple candy melts and swirl with a toothpick. Tap the pan against the counter to help the purple set.
Drizzle with blue candy melts, swirl, and tap against the counter. Immediately decorate with sprinkles, letting some of them cluster together like galaxies.
Place sheet pan in the refrigerator for about half an hour, or until chocolate has set. Break into pieces.