Hello Bess,
i have not the experience to suggest any reason why this was blackened.
But there is a very good website about the badges of the York & Lancaster Regiment, and they show also a die struck, slidered regimental badge from the WWI era and give us this comment:
"It has been suggested that this could have belonged to a Commissioned OR who has converted his badge to an Officers pattern. This badge can be dated by the length of the slider (shorter version 1906) and this particular pattern of badge with sweat holes which was not made after 1918".
Perhaps this happened to your badge ? No OSD-badge available on the frontline, so the OR´s badge was darkened.....
I have also something like that in my artillery-drawer, but it is a complete fake !
Is it reasonable to fake this variant ? Seems to bee ......
An odd ball fake with fake Gaunts mark - and bronzed.
Kind regards
Markus