In a large non-stick skillet,cook beef,onion on medium-high heat until meat is no longer pink. Drain off fat. Stir in water & taco seasoning;bring to a boil.Reduce heat;simmer,uncovered 2 minutes. Stir in mild green chilies,black beans,refried beans and tomatoes. Simmer uncovered for 10 min.
Heat oven to 350 degrees. Place 2 tortillas in a greased 13 X 9" baking dish. Spread half blackbeans and beef mixture;sprinkle w/ 1 c. cheese. Repeat layers. Top with remaining tortillas and cheese. Sprinkle sliced black olives on to top.
Bake for 25-30 min,or until cheese is melted. Add sour cream to each individual servings, if desired.