Add blackberries and lemon juice to a small saucepan.
Simmer on medium heat for about 20 minutes. Stir often.
Strain the pureed blackberries in a fine mesh strainer to discard seeds.
Allow to cool to room temperature.
In the large bowl of a stand mixer or in a large mixing bowl (if using an electric mixer), mix the softened butter and cooled blackberry puree.
Add one cup of sugar, slowly, to the wet ingredients and beat at low speed.
Once the icing sugar is mixed in, add another cup of sugar and beat at low speed. Adding one cup at a time and beating on low prevents the sugar from flying everywhere. Continue to do this with the remaining sugar. Use a rubber spatula to scrape down the sides of the bowl as needed.
Add salt and increase to medium speed.
Mix in vanilla extract.
Beat at medium-high speed until smooth and fluffy.
With a rubber spatula, press the frosting against the sides of the bowl to eliminate any air bubbles.
Spread or pipe onto your favorite cupcakes, cake, cookies, or macarons (when they’re completely cooled).