Is it possible to design a game in Blender? I was wondering if it´s possible to design a game in blender?

For example, can you design a level in Blender then export it to Unity?

It would seem to be easier to do it that way since Blender is dedicated to modelling and the interface in Unity looks quite complex.
Yes, you can do that. You can model every element of your game in Blender then export them into Unity, or any other game engine. The game itself would still have to be coded in Unity.

Blender 2.79, the current version, even has an internal game engine allowing you to design games that works in Blender itself.