That looks like the grease gland, it's located just above their bottom perhaps where you might expect a tail to go. On some piggies they're more predominant than others. This grease gland looks a little bit mucky so perhaps some coconut oil could clear it up a tad to prevent an infection. It just loosens the grease so it can washed out.
I assume that you mean a dark patch at the bottom end of your piggy? your picture and its orientation is somewhat confusing?
If it is at the bottom end, then you are looking at a dirty or overactive grease gland. Please see a vet if the area is inflamed or if there is growth that cannot be removed.
Here are tips on how to clean a grease gland: Boar Care: Bits, Bums & Baths Bathing (including cleaning grease glands)
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