Shanks had the scar in chapter 1, which was around 10 years ago. Blackbeard joined WB's crew at 12yo, so around 26 years before killing Thatch/Marineford War. He also met Buggy/Shanks back then and they didn't interact, plus Shanks doesn't have the scar in the Roger/Oden flashback from 25 years ago or at Roger's execution. So it was after BB joined WB, and after Roger's crew disbanded, but before Shanks met Luffy.
I think the Ace Novel has more about BB.