I've been saying for a while now that the Koby vs Boa fight was off screened and will show that Koby hit Boa in the head and that is where she got the mark from. Along with showing Koby dodge her attacks which even Yamakiji didn't do so toxic mfers can stop saying Koby is gay thus immune to Boa.
plus we can see the page and a panel where Ray confront BB that she didn't had the mark again.
and how about Bandages on her head? Oda also did it to other character
if i recall Wyper didn't even got hit on his head. but he still got a bandages on his head after war / fighting Enel
Thats just how Oda or any author Depicts Character that they are injured.