I think a supernatural source would be more exciting. Vampires who want a permanent way to avoid turning into ashes from daylight? (Though I believe this was a storyline in Skyrim: Dawnguard.) Or a demon who wants to create another dimension of hell, using the Earth as a preexisting "clay" to form it?

And honestly, a sci-fi would be pretty difficult to pull off. For one thing, if the sun was blotted out, every living thing would freeze to death, so you'd have to come up with a reason why that doesn't happen.