Blessing is a technique in SLASH'EM and SpliceHack. It allows the player to change the BUC status of one item from cursed to uncursed, or from uncursed to blessed, similarly to dipping the item in holy water.
The technique is reusable after between 500 and 1000 − (10 × techlevel) turns.
This is one of the techniques that can be seriously abused with the LIMIT mechanics. Get your own HP low enough to trigger the LIMIT and spam blessing until all the items you want are blessed. As long as you have a safe place to do this, a Priest has no need at all for holy water.