SPOILER ALERT
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I just beat the vampire questline. I really wanted to blot out the sun, but it seems that that option does not exist. Does any one know if it's possible to blot out the sun and make Skyrim a haven for vampires? Thanks in advance!

You can't !
That's a great thing !

If you have elven arrows...talk to Serana she'll coat them in her blood so if you fire it with auriel's bow at the sun it blocks out the sun temporarily (for a day supposedly).

All things vampiric spawn in the wilderness attacking people on the roads and near towns but personally for me since i sided with the vampires they are friendly with me.

Yes. I'll confirm that it will block out the sun for one day or rather for the remainder of the day that you shot the sun during. When the sun comes up the next day it will be back to normal again.