Test the consistency of your frosting with a spatula to see if it’s easily spreadable. If your black buttercream is too thick, add more heavy cream (1 Tbsp at a time). If the frosting is too thin, add in more powdered sugar (1/4 cup at a time).
Add this melted frosting back into your mixing bowl, and turn the mixer on again to combine. Continue to heat small amounts of frosting to mix it and add back to your main bowl until you have your desired shade of black.