I’m not sure what to do about getting it authenticated but I don’t want to put it in the mail. I use to have the ghost stamp one as well so I know it’s not that. Any information or comments would be appreciated.
I think it’s just dirt.
Dirt edition 
Can you zoom your phone camera through the lens of a loupe for better pics of the card?
Looks black but would be beneficial knowing if its magenta. Def looks like dirt until you take some meaningful pictures.
pulled a giratina v alt art from lost origin with a white splash on it (like yours but all over the back) of a otherwise perfect card. looked really cool. i assume it was ink from the printer.
This was actually not super uncommon for that card. When I was buying Giratina alt arts, I had to make sure I closely looked at the back of each card to make sure I didn’t miss it. Got burned once or twice when I wasn’t careful enough.