Blackout

Something tells me it's not referring to this.

Does not require you to get the killing blow. I've been getting credit for any HK of an orb carrier.

This is a PVP achievement which can only be earned in the battleground [zone=6051].

The Temple of Kotmogu is a 10-vs-10 battleground released with Mists of Pandaria where opposing factions fight to control four coloured orbs. To win the battleground you need to control the orbs; to earn this achievement your team must kill members of the opposing team who are holding an orb.

To find an orb carrier look for a coloured pillar of light which will stay above their head as they move; carriers tend to stay towards the middle as their team earns more points when they are closer to the center. As mentioned elsewhere you don't have to land the killing blow, kills made by your team will count towards this achievement.

This achievement may take time to earn but essentially if you keep joining this battleground you should eventually get this one completed.