The Blade reference line is the line that goes through the Aerodynamical reference point of the airfoil of each of the stations composing the blade.

For a Straight blade , by definition, the blade reference line is the Pitch axis .
For a Prebent blade, the blade reference line is defined by the BlCrvAC and BlSwAC parameters of the Blade shape file .

The figure below illustrates the blade reference line for a blade with an exaggerated in-plane prebend. In this image, the Aerodynamical reference point is in the middle of the chordline.