The Staining Properties of Blackberries

In the final Poirot short story, a major plot point hinges on the fact that blackberries apparently stain one's teeth black or at least significantly purple. I asked all of you to write in if you had any knowledge to share on the issue. And indeed you did not. So here's what one of you wrote: We think it might have been like icees slash water ice are today. They stain your mouth for a while side. Not but this is very true. As a teacher of British literature, my team students and I are already calling HM TQ the old Queen. Charles is the new king.