Stir together sour cream and horseradish in a small bowl.
To assemble, spread a tortilla evenly with 1 heaping tablespoon of the horseradish mixture. Cover with watercress and a quarter of the roast beef, making sure the ingredients don't quite touch the edges. Spread another heaping tablespoon of the horseradish mixture over the top, followed by a quarter of the apples and 1 tablespoon of blue cheese. Roll up the tortilla. Complete the wraps with the remaining ingredients. Cut in half on the bias.
These wraps can be made a day ahead, sealed in wax paper or foil and refrigerated. Serve cold or at room temperature.