In a small bowl, combine the oil, soy sauce, sugar, and cornstarch to make the marinade. Stir until smooth.
In another small bowl, prepare the sauce using the same ingredients. Coat the beef fillet with the marinade and refrigerate for 15 minutes.
Heat a wok over high heat. Add the beef and stir constantly for 5 seconds to prevent sticking. Remove the beef to a plate and let drain.
Pour off all but 1-2 tablespoons of oil from the wok. Return the wok to high heat. Add the paprika and stir-fry for 10 seconds. Add the garlic and black beans, then pour in the prepared sauce and bring to a boil.
Return the beef to the wok and stir-fry for 5 seconds.