You need to blacklist the radeon driver, so that fglrx is loaded instead.
- Bug#765109: fglrx-driver: Black screen after xorg startup Vitor Antunes
- Bug#765109: fglrx-driver: Black screen after xorg sta... Mathieu Pourcel
- Bug#765109: fglrx-driver: Black screen after xorg... Vitor Antunes
- Bug#765109: fglrx-driver: Black screen after xorg sta... Mathieu Pourcel
- Bug#765109: fglrx-driver: Black screen after xorg sta... Vincent Alquier
