Preheat oven to 375°. Melt 3 tbsp. butter in a large frying pan over medium heat. Add pears and salt. Cook, stirring occasionally, until pears are soft, about 10 minutes.
Meanwhile, cut baguette into 16 slices (1/4 in. thick; see Notes). Spread slices with remaining 1 tbsp. butter and arrange on a baking sheet in a single layer. Bake slices until well toasted, 10 to 15 minutes.
When pears are soft, stir in sage and remove from heat. Top toasted baguette slices with pear mixture and blue cheese. Sprinkle with pepper. Serve warm.
Note: Nutritional analysis is per crostino.