For blades, the twist referes to the rotation around the Blade reference line of blade stations It therefore does not apply to the whole blade (as opposed to the Pitch). A 0 degrees twist corresponds to the chordline of the airfoil composing the blade station being parallel to the rotor plane. The positive direction is clockwise when looking from the tip towards the root.

Note: this parameter is defined for each station in the Blade shape file.

The image below illustrated the twist angle for different configurations:

in the left image, the twist of all blade stations is 0 degrees
in the middle image, the twist of all blade stations is 45 degrees
in the right image, the twist is 0 degrees at the root of the blade and 90 degrees at the tip

The twist angles along a blade never changes during operation. It is the relative twist angles of the blades station that is of importance, but by convention the outer most station of a blade (tip station) has a twist of (approximately) zero.

Examples:

The blades of NREL 5MW ref. WT has a tip twist of 0.106 deg and a root twist of 13.308 deg.
The DTU 10MW ref. WT has a tip twist of -3.428 deg and a root twist of 14.500 deg.