It’s just where the gold braze has patina-ed. Braze and copper loops very often oxidise to black on Victorian badges.
You can see verdigris and darker spots on the right scroll adjacent to to the attachment point and braze which has turned dark. Also a spot of verdigris on the braze at the bottom.
Clearly at some stage in its life these these areas of the badge were expose to damp and have tarnished and started to corrode.
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