Preheat oven to 350 degrees F (175 degrees C).

Heat vegetable oil in a large skillet over medium heat.

Add chopped onion to the skillet and cook until softened, about 5 minutes.

Stir in diced tomatoes, tomato sauce, and minced garlic.

Simmer uncovered until flavors combine, about 30 minutes.

Divide refried beans, 1/2 cup chopped onion, 1/2 cup black olives, and 1/2 cup Mexican cheese blend evenly among the flour tortillas.

Roll up each tortilla to enclose the filling.

Arrange rolled tortillas seam side-down in a shallow baking pan.

Pour tomato sauce evenly over the tortillas.

Sprinkle the remaining 1/4 cup black olives and 1/4 cup Mexican cheese blend on top.

Bake in the preheated oven until the filling is heated through, about 30 minutes.