You can either prepare all the crostini yourself or just prepare a few as an example and let your guests prepare them themselves. I prefer the latter option, since it takes less time for you, requires less space to serve, and gives your guests something to do with their hands while you sit around and chat before dinner.
Spread the tops of a few crostini with brie and top with 2-3 blackberries and some freshly ground black pepper. Serve prepared crostini on a cutting board alongside the rest of the brie and a cheese knife. Arrange remaining crostini slices and blackberries in bowls and serve with appetizer plates.