The ability triggers after you take the damage so you do get the up to zero then heal effect. I don't see how you would get past 50% health when you are gaining 25% of your hp.
Because Immediate Interrupts are resolved before the action that triggers them resolves, and may invalidate the triggering action completely.
As an example, the Wizard power Shield is an Immediate Interrupt that raises the wizard's AC and Ref by 4 for a time. The reason this power is awesome is the interrupt... something attacks you, and then you use the power. If it gives you enough AC that the attack misses, then the attack becomes invalidated and you've traded an immediate action for a monster's standard action--an EXCELLENT trade for an encounter power.
As an extreme example, let's say your Warden is up against a monster with an minor action that says after its range and all that: 'Can only target a bloodied creature, 5d6+9 damage, plus 3d6 on a crit.' A dickish power, but hey, sometimes dickery is good on a solo.
Monster rolls a crit, and does, say, 50 damage. Warden's got 80 hp max, setting his bloodied at 40, and his surge value at 20. Let's give him swift recovery, make him a dragonborn... so we're looking at probably a surge value of 27 realisticly. BUT! He's at 19 hit points.
How does this all work out?
Well, the 50 damage knocks the warden down... but he uses his immediate interrupt, so time backs up to before the action starts. He's healed from 19 to 46 hit points... thusly he's no longer blooded.
Now you re-resolve the offending action from the very beginning. Target: Blooded creature. The power has an illegal target! Thusly it is invalidated. The damage never occurs.
'But how can you back up in time!!?!? What!?!?' That's what the words 'Immediate Interrupt' mean. It means whatever happened... this happens before it. They're the quick reactionary things people do just before something smacks them. Interrupts happen first. That's what they do. And yes, they CAN invalidate actions... that's actually the term the rules use for that sort of situation.