In a saucepan combine blackberries and lemon juice. Using a spoon, mash the blackberries and let them simmer for 7-8 minutes. Once done, pour the mixture into a wire mesh sieve to remove the seeds and pulp. Set the blackberry syrup aside to cool down.
In a stand mixer with a paddle attachment, beat the butter on medium speed until the butter turns light, usually 3-4 minutes.
Change to whisk attachment and add salt. Add confectioners sugar in batches and beat the buttercream on medium speed until smooth and fluffy, 6-7 minutes. Add the prepared blackberry syrup and beat for another 2 minutes, until well blended.