For taking pictures of concerts two options are possible. With or without flash. Personally, I prefer without flash. Why? Because it keep the authenticity of the colours with the movement I cherish so much.
Now the difficulties lays in the settings and the technics. On a major concert scene, you might have sufficient light produced by the spots to have a good outcome.
However, in smaller concerts it may not be the case. In this case, the choice of ISO is, of course important. High ISO will facilitate your work. In the case of the concert of the Discrepancies, I mostly used 1600 ISO. In any case, a picture treatment will be necessary to enhance them and reduce the noise for example.
The second important setting is a high opening. I used two lenses and one was a 50mm with a maximum opening of 1.4. This helps to obtain as much light as possible but creates another parameter to manage which is the depth of field, more difficult to handle on a moving character.
You also have to make sure to have a steady position when you press the trigger, even when in the movement.
You can see in these pictures that 2 outcomes are good in my perspective. Vivid colours and movement that provide a better feeling of the event.
Last point, in a concert it is difficult to have a good picture of a moving character. Closed eyes, wrong position, etc. So I recommend to shoot in continuous mode. Yes, you will end-up with hundreds of pictures, but this increase the probabilities to have a good picture.