Think that you already answered your own question. If this is a wide scene then crop that singer out using PiP Designer. You may want to be creative and make a tighter crop when the other singer is too close to the middle singer and vice versa using keyframes.
You may want to only blur the unwanted singer and the others using something like the bokeh effect using radial blur. See this demo:
https://www.youtube.com/watch?v=oibzeyWXkvc . The mask is part of the same fx.
If the singers move all over in the video then the motion mask in Colordirector may be needed.