Best extension ever! Exactly what I was looking for!

The extension claims to only block Reddit, but it also blocks the following sites:

tweakers.net
tweakers.nl
hckrnews.com
ycombinator.com
imgur.com
xkcd.com
youtube.com
smbc-comics.com
oglaf.com

There is no way to change this without editing the source. There is also no indication that this extension broke these sites - they just show a blank screen when you try to use them. Seem to work OK apart from that, but you need to be able and willing to edit the source code manually.