Roller blinds can be fitted either inside or outside the window recess, it merely depends on your personal preference.
When fitted inside the recess, they are closer to the window and provide a neat look. The only downside is that they could restrict the amount of light that enters through the window.
Outside the recess, the roller blind will be further away from the window, allowing light to flood into the room when raised. When lowered, it will block out light.