Combine first 3 ingredients in a bowl; toss with a fork until moist. Press into bottom and up sides of a 9-inch springform pan coated with cooking spray.
Combine 3 cups blackberries and fresh lemon juice in a blender; process until smooth. Strain mixture through a sieve over a bowl; discard solids.
Place salt and egg whites in a large bowl; beat with a mixer at high speed until foamy. Combine sugar and 6 tablespoons water in a medium saucepan; bring to a boil. Cook, without stirring, until a thermometer registers 250°. Gradually pour the hot sugar syrup into egg white mixture, beating at medium speed, then at high speed, until stiff peaks form. Gently fold in blackberry mixture; pour into prepared crust. Cover; freeze 8 hours or overnight. Let stand 5 minutes at room temperature before serving. Garnish with fresh blackberries and mint, if desired.