For the sauce, set aside 4-6 blueberries for garnish and place the remaining blueberries in a small heavy-based pan with the wine or sherry and sugar.
Simmer over a low heat for about ten minutes, until the blueberries reach a mushy consistency. Add the vinegar, orange juice and salt and freshly ground black pepper to taste. Stir in the butter and keep the sauce warm.
While the sauce is simmering, prepare the venison. In a frying pan, melt the butter and oil over a medium heat and add the venison to the pan. Fry the venison for 3-5 minutes on each side, until cooked to your liking. Leave the venison to rest in a warm place for at least five minutes before serving.
Place the venison steaks on two plates, spoon over the sauce and garnish with the remaining blueberries. Serve straightaway.