The present invention relates to a drilling tool and method for modifying a blind hole in a blade root insert of a wind turbine. The invention further relates to an apparatus and method for cleaning a blind hole. The method for modifying a blind hole comprises: removing a blade screw located in the blind hole, inserting a drilling tool into the blind hole, fixing the drilling tool in a drill, recessing the blind hole by a predetermined depth, with a maximum diameter of the end section and of the hole bottom after recessing being smaller than or equal to a diameter of the end section prior to recessing and a transition from the end section to the hole bottom being round after recessing, removing the drilling tool and the drill, mounting the blade screw in the modified blind hole.