Duude007 there is no explanation that beacuse lack on the BLessing of Light, a self sacrifica was required to create one. What Minfilia said, where just suggestions, that maybe was that maybe not, of the situation of those events she couldnt be sure, no one could. The Blessing of light was like a shield, that Hydaelyn gave the WoL in order to come some inmunity to some powerfull atacks than on those times we couldnt be able to resist, but that changed. ( The fack that we defeated Nabriales withouth the blessing confirms that.)
It was Hydaelyn who gave the WoL the power to create the blade of light, but if a mortal being wants to re create such a weapon withouth a god's help its when a self sacrifice would be required I suppose.