Continuity mistake: When The Bride cuts Sophie's arm off you see her get covered with blood. In the next shot, however, there is considerably less blood on her top and it is in a different pattern. (01:11:20)
Continuity mistake: When The Bride and Vernita Green are in the kitchen, Vernita mentions something like "I've got to fix my daughter's cereal." Notice when she says this, The Bride is holding her mug with the handle facing Vernita, when the shot changes, it is instantly facing The Bride - something which cannot be achieved this quickly.
Continuity mistake: In the anime sequence after O-Ren Ishii has drawn the sword out of Boss Matsumoto, the blood is splashing out of him, and up on her face. But the amount of blood on her face keeps changing. (00:40:00)
Answer: From what I understand the reason for the lights being off is the same reason for why the previous scene was done in black and white; to decrease the amount of 'graphic violence' in the movie in an attempt to keep an 'R' rating. I would assume that they had him shut off the lights for that scene as just another method to accomplish that task.
I believe the original question was asking why was it done within the context of the film (i.e. why did the character shut off the lights) not why was it done in reality. My best guess is that the manager switched off the lights thinking the 88 had a better chance of killing the bride if she couldn't see. True, they couldn't see either but there were so many of them one could possibly have gotten to her.