Bring 200 g blueberries with lemon juice, powdered sugar and cassis to the boil in a small saucepan and simmer for 4 minutes over a mild heat. Strain through a fine sieve and squeeze out as much juice as possible. Add the remaining blueberries and let them cool.
Mix the buttermilk with honey and lemon zest until smooth and mix with the cold blueberry compote.