The blast from an artifact is doubled if the artifact deals double damage to your current form (e.g. if Ogresmasher blasts you while you’re polymorphed into an ogre, the blast damage is doubled).

Quest artifacts, and probably all artifacts, blast other monsters when they enter their inventory, subject to the same checks and chances as the player (however, the artifact will never evade their grasp). This is most likely to be observed when a quest nemesis or the Wizard of Yendor steals a quest artifact from the player.