Melt chocolate in a glass bowl in the microwave, or in a metal bowl set over a pot of simmering water on the stove. Stir frequently until melted and smooth. Remove from heat and stir in butter until melted. Line a baking sheet with waxed paper. Add blueberries to the chocolate and stir gently to coat. Spoon small clumps of blueberries onto the waxed paper. Refrigerate until firm, about 10 min. Store in a cool place in an airtight container. These will last about 2 days.