For those who don't know, to "Black Out" means that your whole team fainted.
If I use Thief on Red's Pikachu and steal it's Light Ball, then he proceeds to sweep my team to nothingness, do I keep the Light Ball??
No answers saying "maybe" or "possibly"
Inspired by JirachiCelebiMew's comment on this question:
How can I get the light ball in heart gold without the event, or if not how can I get the event?
You will keep the Light Ball, as the action has been done already. This is only for Gen IV and below. - Experience with Level 60's with Red
For Gen V, your stolen items are returned to the trainer, so it will only work for Wild Pokemon. - Source