It is overexposed. Your camera metered the black areas of the scene as medium gray. That caused the shutter speed to be too slow and you also got motion blur. Concert photography is one of the most demanding types on several levels. Getting proper exposure in such an environment is as much an art as a technical skill, and takes a lot of practice to find the best way you can get it.

[Here's one way I prefer to do it.][1] 

[Here's another][2] that is more applicable to the shooting situation in your question.

> I was all the time in Aperture mode, maybe I should have switched to Shutter priority mode?

In any automatic mode (P, S, A) you're going to need to use some exposure compensation to prevent the camera from trying to make the black background areas too light. Ultimately you need to learn to shoot in manual exposure mode for best results in such a challenging setting. Regardless of which shooting mode you are in you need to pay attention to the shutter speed and aperture values displayed in the viewfinder and adjust accordingly. Using spot or center weighted metering also helps for some folks. I prefer Evaluative (Canon)/Matrix (Nikon) but I've learned to see how much *EC* I need.


  [1]: http://photo.stackexchange.com/a/40762/15871
  [2]: http://photo.stackexchange.com/a/73479/15871