Then pour into a small sauce pan. Heat the mixture gently, stirring often, until hot to the touch.
Let cool for a few minutes, then pour into the heart molds.
Repeat with the remaining colors if desired, washing out the blender and sauce pan in between colors. If you only are doing one color for all 12 cakes, just multiply the amount of coloring ingredients by 4, so like 3 tbs of Blueberries would be 12 tbs (3/4 cup).
Put in the fridge and leave until set, at least 4 hours.
Once set, remove from the molds. Then use stamps, a toothpick, or a knife to make the words if desired.
Microwave the blackberries for about 20 seconds or heat on the stove to release the juices, then push the juice through a fine-mesh sieve into a small bowl. Using a fine-tip paintbrush, paint the words using the blackberry juice.
Store in an airtight container in the fridge until ready to serve.