For the cheese sauce, in a medium saucepan over medium heat, combine flour, salt, onion powder and black pepper. Cook for about 2 minutes, stirring constantly. Slowly whisk milk into the pan; add cream cheese. Whisk vigorously, until even and no lumps appear. Bring to a low boil and reduce heat to a simmer. Cook about 4 minutes, stirring, until thick enough to coat the back of a spoon. Remove from heat and stir in cheese until melted.
Preheat oven to 400ºF. Arrange tortilla chips on a baking sheet. Top with black beans, tomato and scallions. Bake 10 minutes.
Mix salsa and cheese sauce. Pour over nachos. Top with lettuce and squeeze a small amount of sour cream in a zig-zag pattern all over the nachos.