I was discussing this with Flammos earlier today, but the discussion didn't really bring anything to fruition. I believe that the Blood Knights can harness the holy magic of the Sunwell based on this quote;
As of the end of the Burning Crusade expansion, blood elves who wield the Light do so through the power of the renewed Sunwell
Source
Flammos argued that that wasn't CotH lore, based on
Tales' Guide.
I'm of the opinion a Blood Knight is open to either option, but would like to know the server lore on this.
Personally, I thought that this was how all post burning crusade expansion blood elves got their powers in the light. It is the means I use for my blood elf also at least.
I always said that Krian uses the Light from the Sunwell, as he doesn't directly believe in the Light. He used the powers of the Naaru before the Sunwell.
... Hm. I believe they can go either way.
Either way, but I'd doubt the light would come to a still-fel-infused creature naturally (non-sunwell) unless it was a very firm and/or zealous believer.
In my opinion, either way is possible.
Otherwise the simplest way to disarm a paladin/priest would be to drop a dab of fel blood in their mouth.
EDIT: Though, I do agree with Dave there. One must be very firm in their belief of the Light to wield it as a Sin'Dorei. Moreso than other races.
Tales' guide is not CotH lore.
It's a guide to paladins.
Besides, the whole Blood Knight thing was completely ignored in Wrath and only retouched in Cata. Which was well after the guide. Which makes it outdated.
Plus, the Sunwell is a power source all on its own. The Sin'Dorei tap into it to sate their magic addiction. It's not that far of a logical leap to assume BKs still get their power from the Sunwell.
Plus, Blizz lore > CotH lore.
The vibe Blizzard offers on this is more than a bit ambigious, but I have not read anything in-game that suggests that the blood knights can actually just channel it from the Sunwell.
Though I imagine since the blood knights as a faction -all- joined under A'dal and the Sha'tar I'd imagine they'd all became lightworshippers.
Blizzard has definitely said that the Sunwell is different now and will help the Elves improve over time. Also consider that Tales' guide to Paladins was written well before we went past the Sunwell plotline on the server so the above post referencing it is not accurate. That said, Elves may or may not believe the Sunwell/Naaru/etc. really has become the source of their power. Who knows?
Also you seem to like thread necromancy.
I'd just like to note that this thread is rather old and the issue was resolved quite a long time ago. This is simply a case of thread necrosis, as it is.