User blog comment:Blood Reaper/Skyrim Dawnguard Trailer/@comment-3502822-20120601154436/@comment-76.14.103.94-20120601175706

I would guess that the falmer are working with or for the vampires. If you watch closely there is a falmer killed in the trailer. As for blotting out the sun, because the falmer are blind they don't lose anything from it but it definitely gives them an advantage against all of the people who do see.