Place the blackberries and lemon juice into a large saucepan over medium heat. Cook, mashing them with a masher, for about 5-7 minutes.
Strain the mixture through a fine wire strainer. Set aside to cool.
Combine the zest and white sugar with your fingers until well combined and the sugar is fragrant; set aside.
Cream the butter with a hand mixer until creamy and smooth. Add the fragrant sugar and blend until well combined.
Add the eggs, one at a time, and mix thoroughly. Add ½ cup of blackberry juice and blend until well combined.
Pour into a small saucepan and cook over medium heat, stirring often, until thickened, about 10 minutes at about 170 degrees. Remove from heat and let cool before pouring it into a container. Side Note: If you accidentally overcook the sauce, it may curdle a bit. Simply pour through the fine mesh strainer to remove any clumps.
Serve over crepes, pancakes & waffles, toast, ice cream, muffins, or sugar cookies.
Store in an airtight container in the refrigerator. Enjoy.