Magian staff or SoulscourgeFollow

I've never heard of them having a hidden effect that boosts the damage of magic bloodpacts. With Magian weapons, what you see is basically what you get (Relic and Mythic weapons are exceptions as far as having secrets to them, but that's because of the weapons themselves and not the Magian trials). The Magian -Perpetuation staves simply allow you an easy way to cap out your Bloodpact Ability Delay down timers as well as allow you to equip things other than -Perpetuation Cost for your other slots if there are better options available. For example, while you've just got your avatar out meleeing, you could use Summoner's Bracers +2 instead of maybe Adhara Gages or use Summoner's Doublet +2 instead of Caller's Doublet +2. The Magian staves just free your gear up to allow for other options for what you can equip for your avatar.

While Soulscourge falls short of the Tuahjebat line of staves as far as +Attack or +Magic Attack for avatars, it is a good all in one option, and should be what you're using for Bloodpact: Rages (unless you have the aforementioned Tuahjebat staves or a Nirvana) as well as Bloodpact: Wards (unless you have a Kirin's Pole with +Smn skill).

Nirvana has no hidden effect to it. What you see on the weapon is what you get. It is the best -Perpetuation cost staff available, and the best for magical Bloodpact: Rages. However, you will still want Soulscourge or the Fire path Tuahjebat for physical Rage Bloodpacts.

That's a given boost to all Relic and Mythic weapons, that it boosts the damage of the associated weaponskill from Lv. 90 onward. As far as hidden effects that would affect your avatar though (which is what I was meant when I said there wasn't one), there are no hidden effects to it. What you see is what you get.