Combine 400 g blueberries, lemon juice, powdered sugar, and cassis in a small saucepan. Bring to a boil over medium heat, then reduce to low heat and simmer for 4 minutes. Strain through a sieve, pressing gently to extract as much juice as possible. Stir in the remaining blueberries and let cool completely.
Stir together kefir, Greek yogurt, honey, and lemon peel in a bowl until smooth. Pour the mixture into 4 deep plates. Place the cold blueberry compote in the center of each plate and use a fork to create decorative swirls.
Optionally sprinkle crumbled amarettini over each bowl before serving.