Ah-hah! I've found it!

Valerica states that:
"If Harkon obtained Auriel's Bow and Serana's blood was used to taint the weapon, the Tyrany of the Sun would be complete." http://www.youtube.com/watch?v=r84W4UATSBY : Time 1:08:00

Serana's blood was really suppose to be used to taint the weapon itself, not the arrows. A quick google of "taint" brings up the verb definition of "Contaminate or pollute (something)". So Harkon would need to coat or bathe the bow in her blood, thus making the sun-blocking effect permanent...which would likely kill her...because he would need a lot...if not all of it.

I'm still looking to see exactly when and where they talked about the arrows being tainted. I know that the snow elf in the Forgotten Vale offers some information, but I'm thinking that someone might have mentioned it before then....so I'm keeping my eyes and ears open as I go through my Second Play.

Why do you need to know this, you might ask?

You don't.

Why post this information then?

A while back in another thread (which one eludes me) there was some argument about the tainted arrows and the effect duration. I knew that someone said something to this effect, but at the time I couldn't remember where...and on my second play-through I came across it...so I thought I'd share the info with you all.

Why can't Harkon just take some blood and than heal Serana with a healing spell than repeat till you have enough blood?

I noticed the anomaly when Valerica first speaks it. You are right...she does state it's the weapon, not the arrow.

And the blood arrows apparently do diddly squat on people, as far as I can see...

I know! I was really hoping that they'd send other Vampires into a BLOOD FRENZY or something.
...maybe morph creatures into something hideous.
...or accidentally summon a gargoyle to fight you while teleport your foe somewhere else.
...or just cause a multitude of random effects like corrupting the target or summoning a small blackhole...


I was disappointed when they did nothing against foes.

All they did for me was cause Harkon to get angry...then ctd and next reload I bashed him with a sword....which was no easy thing just quietly when he kept doing the bat thing and Serana was ice spiking and draining me from behind.