The berries are called barberries, Berberis vulgaris. If found them in some nice bushes in the street. You can see them in these pictures.
As you can see, their peel (their skin) is blue. But their juice is red. It's amazing to see how the ink changes its colour when you write with it, depending on the kind of paper you use. Sometimes it remains bright red, others it turns purple. And if the paper has too many chemicals, it becomes dark grey.