Line baking sheet with paper toweling. Slice tops off tomatoes, reserving tops; remove stems. Using melon baller, hollow out tomatoes, discarding seeds. Invert tomatoes on paper toweling; drain.
Blend cream cheese, blue cheese, mayonnaise, milk, salt and pepper in bowl; dont blend totally smooth. Add pistachios. Chop reserved tomato tops; add to cheese filling.
Spoon cheese mixture into large pastry bag (without any tip). Pipe into tomatoes. (Or use small spoon.) Chill until serving. Makes 12 servings (2 tomatoes).