The invention relates to wind turbine blade handling aboard a vessel. According to the invention, a method for handling wind turbine blades (10) aboard a vessel(100) comprises: providing on the vessel a blade rack assembly (1) configured to accommodate more than one blade, the rack assembly comprising at least a root rack (2) and a tip rack (3), wherein the root rack and tip rack between them define a blade support plane (18); providing a jack (30) acting between the vessel and one of the root or tip rack; and raising or lowering one of the root or tip rack aboard the vessel by means of the jack to thereby move the blade support plane through an elevation angle [theta]. A jack assembly on a wind turbine installation vessel (100) capable of raising or lowering a rack of wind turbine blade root or tip support frame elements (4, 5), the jack assembly comprising a lifting platform (34) driven by a drive device (32) and being positionable at a rack (2, 3) to be raised or lowered. An offshore wind turbine installation vessel (100) comprising a loading deck (11), a crane (120) and a jack assembly capable of raising or lowering a rack of wind turbine blade root or tip support frame elements (4, 5).