Gently mix black beans, avocado, pico de gallo and lime juice; set aside. In a large cast-iron or other heavy skillet, cook potatoes and crumble sausage over medium heat until sausage is no longer pink and potatoes are tender, 6-8 minutes.
Whisk together eggs and milk. Pour into skillet; cook and stir over medium heat until eggs are thickened and no liquid egg remains. Stir in cheese. Spoon egg mixture into tortillas; top with black bean mixture. If desired, serve with sour cream, cilantro and additional pico de gallo.