Well I think it fits better on "Circus" cuz I feel "Blur" a bit more organic than "Blackout", I mean I don't feel the electropop vibes of Blackout on "Blur".
I think "Kill the lights" fits better on "Blackout", Actually "Kill the lights" sounds like a really bad track that was far away of making the final cut on the epic "Blackout"
To me, the vibe of Blur is more urban and completely fits Blackout. Should have been listed after perfect lover and before Why should I be sad?
I do agree that Kill the lights fits the blackout theme as well but as a bonus track.