TBQH, I handwave it, as I know it’s make-up. That said, I’m sure somebody’s got a better explanation.
Folks?
What if it was an indication of being ‘in season’. Females seem to have a blue highlight on their head-spoons, so whether it’s makeup or natural, it could be a sexual advertisement like flushed lips/lipstick.
Alternatively, but in the same ‘sexual signal’ vein, it could be a scent gland or a vestigal scent gland? Either way, the blue pigmenting of it seems meant to draw attention, and in nature that’s either to redirect from a vulnerable area–which the spoons don’t seem to, being on vulnerable spots–or as a sexual display. So as a naturalist I’m pretty convinced it’s a sexual display that had some kind of fertility meaning way back in The Wild, but has since developed into a beauty standard.