The Blade length that is given for example in the Blade database or in the Information pane of the Blades part corresponds to the projection of the blade onto the Pitch axis. Note that this is the blade length at the beginning of the simulation (i.e. t = 0), as the blade is generally deformed during a simulation and its length can vary.
The blade length is different from the length of the Blade reference line for prebent blades. The figure below shows the NREL 5 MW blades with a large in-plane prebending to illustrate the difference between Blade length and length of the blade reference line.
In practice, for realistic blades, the difference between the Blade length and the length of the blade reference line is very small. For the DTU 10 MW blade for example, the difference is about 0.1%.