Preheat the oven to 375 degrees F (190 degrees C). Butter 2 baking sheets.

Cut crusts off bread. Cut bread into 1 1/2-inch cubes; set aside.

Melt butter in a large saucepan over medium heat. Stir in cream cheese until melted. Remove from heat; stir in Cheddar and blue cheeses until melted.

Beat egg whites in a small bowl until stiff peaks form. Fold into the cheese mixture. Dip bread cubes in the cheese, turning to coat.

Place dipped bread cubes on the prepared baking sheets.

Bake in the preheated oven until bottoms are golden brown, 12 to 15 minutes. Serve warm.