I'm not completely sure if this is the same dagger, but if you *start* the "Gentle Blade" quest from 'Fabrizio the Master Weaponsmith', he gives you a blessed dagger to go kill his werewolf wife.
So long as you don't complete the quest, the dagger remains blessed in your bag (I believe this is why you want 'Someone's Dagger'). It may be the same dagger, but I'm not sure.
Fabrizio can be located in the Northwest building (Lycaeum) of Moonglow. Very easy to find, and all you have to do is accept his quest to get the dagger.
For the record, completing his quest gives you Misericord - a wolf slayer dagger (though not blessed). Only wolf slayer in the game, I believe. However it is only for fencing, whereas the Blessed quest version is UBWS.