Q: Blue pixel on iMac

One dead pixel is nothing to worry about. If you have a large number of dead pixels interfering with viewing you want to replace the display.

On older matte finish panels, sometimes one could un-stick a pixel by rapidly changing colors and massaging that spot on the panel.

About all you can do these days with solid glass panels, is to rapidly change the color and hope that the pixel un-sticks.

For Apple's take, see > About LCD display pixel anomalies for Apple products released in 2010 and later - Apple Support

Replace LCD as a whole at Authorised Service Provider or Apple Store if it's annoying to you.