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Unless the severly changed the functionality of the skill on console compared to PC, which i dont think they did, Aborb Magic has nothing to do with blocking apart from its passive benefit to block cost and effectiveness.
Activating the skill applies a 30s buff on the player, that will absorb the next magic projectile hitting him, regardless of him blocking or even being on the same bar the skill is slotted on. Then it is consumed, the damage prevented and the player is healed for a certain amount of his maximum health.
It is important to know that the absorbed damage is capped. While we dont know the value on live, on PTS the tooltip actually displays the maximum amount of damage absorbed. It was around 7k for my charackter there, but I am not sure what exactly is factored into the calculation. From what i remember, if the damage of the projectile exceeds the threshild, aborb magic has no effect on that spell. This becomes apparent when trying to absorb meteors for example, or hard hitting crystal frags.
Also keep in mind the ability only works on spell PROJECTILES so a daedric curse for example wont trigger the effect and damahe you like it is not there at all.
On a personal note i found the reflect morph way more useful, especially since it has no cap on the damage reflected and is a great tool to counter hard hitting spell projectiles, such as meteor, frags and overload.
Keep in mind however, that ,regardless of morph, many skilled caster will use a staff light affac right before casting their spell, triggering your buff and making the way clear for their big hitter, unless you instantly recast your defensive posture (or its morph).