mixing cake batter in a blender?
If you have a recipe which calls for "creaming the sugar and oil", forget it and find another recipe. Creaming is a process which only makes sense with a solid fat, so the sugar crystals create tiny air bubbles which are then expanded during leavening. Mixing sugar and oil separately, with or without much whipping, doesn't do anything. If the recipe author was not aware of that, he had no business creating recipes, and you can expect there to be other mistakes. 

But if we assume that you have recipes which require real creaming (they will use butter or other solid fat), you can't make them in the blender. The cutting motion of the blender blade is very different from the paddle action of a mixer. It won't work. You can do it by hand, although it is a bit tedious. 

If you have other cake recipes, for example muffin method cakes, it will work OK. You shouldn't overwhip, for the reasonds SAJ mentioned, but it is not guaranteed that it will happen. A cake recipe has lots of fat and uses a low-gluten flour. If you blend in short pulses, the result's toughness will be very slight compared to a mixed cake, it can cost you points in a culinary school, but family and friends won't notice it. On the other hand, there isn't any special reason to use a blender for them and not mix by hand, as there isn't much work involved there.