Got my vitamin C from a guava (NT50) that was sliced, bagged, and tossed with a dark powder — not plum powder, I don't think, but language barriers prevented an I.D.
Update: It was liquorice powder, a reader tells me. (Thanks, Chung-chieh Shan!)
Guava Vendor
Wufenpu Fashion Center south of Raohe St., Taipei