Bethesda didn't allow us to block out the sun for good because they don't want to HAVE to put it as canon in the future...but to make the Vampires happy we can fire a arrow into the sun to temporarily block it for a day.
Which is a stupid reason.
Bethesda is afraid to step on ANYBODY's toes (
ANYBODY!!!) when it comes to declaring things canon, so they relentlessly cop-out of it. First we had Warp in the West, which was more than ridiculous, and since that was so ridiculous, what's their new tactic? Let's just eliminate all potential divergence from quests so that EVERY character makes choices that leads to the same end result, and thus EVERY possible character can be considered canon.
They're literally compromising gameplay for the sake of people not crying about their character not being canon. You know what? Tough [censored]. Anyone with half a brain should be able to figure out that the ending where
THE SUN IS FREAKING GONE is not the canon ending, and if they really can't figure that out and say "wtf you mean my vampire overlord that killed everyone and everything isn't canon?!! This is BULL****!!111" then you're beyond stupid.
I seriously wish Bethesda would stop catering to such an audience and learn how to say "no" every once and a while. They can easily allow for the sun being blocked out in Dawnguard and then come TES VI, say "tough, it's not canon."