The shield gained per hit benefits from Shield power, but the cap does not.
The shield can be very easily stacked up prior to fights by farming minion waves, allowing the wielder to constantly overheal.
Bloodthirster is a good defensive option when under lane pressure or poke damage, as it allows the wielder to shield from damage and heal back when hurt.
Blood Barrier works at the same time as Overheal, resulting in doubled shielding (respecting their respective caps).
Unearthed near the marshes of Kaladoun, the Bloodthirster is thought to have belonged to a powerful soulstealer named Frax Van Itt. The wielder of this dangerous weapon is forever bound to the blade and its thirst for the blood of others.
Unique Passive - Blood Barrier: Life steal from basic attacks can now overheal you, converting the excess healing into a shield that absorbs up to 50 − 350 (based on level) damage and slowly decays if you haven't dealt or taken damage in the last 25 seconds.
New Unique Passive: Your life steal effects can overheal you, converting the excess healing into a shield. The shield can block from 50 to 450 damage, based on champion level, and decays if you have been out of combat for 15 seconds.