mbp screen goes black on startup

I recently installed snow leopard om my mbp. It starts to load when it turns on.The screen lights up and the Apple logo appears. The timing gear appears and goes through the sequence. The screen goes blue and then goes dark. You can barely see, with the aid of a flashlight the logon account screen in the back ground.It is very dark. I can turn the mbp off and then start back up in the safe mode It comes back up and loads OK. I then have to restart it .It works after that operation