Out LK-ing a Light Mage isn't the problem with them, it's the gigantic boost in MD they get from Mana Shield and Light Shield that reduces Blessing damage way too much to the point where a lv 170 light mage can completely block them against 700~900 LK.
Not to mention they've got % based auto-healing skills that practically cover any damage Blessings manage to get through.
EDIT: I really wish Blessings were way more powerful, so those who don't buy into MyShop and the like can use them without feeling like they're 1st job spells when they see the damage. The issue of them being overpowered is a matter of personal strategy against them a thing this game lacks inbetween the run up instant 1HKO and the Arrow Rush spam. Here's a chance to actually make Arrow Rush a bad idea