the blue plastic should be placed in the main window in front of the semi-reflective mirror so that the direct optical path will be coloured blue and the rangefinder image (which is usually yellow) will be more contrasty against that. however I've never tried that.

sometimes black marker pen can do little magic when you precisely make a small mark/patch on the main window plastic cover (from the front side of the camera) - it helps to tame the direct image so the rangefinder image is better visible. it's especially useful in cases you have a camera that has been inadequately serviced - like someone cleaned the semitransparent mirror before