Berry juice was sometimes used as ink back in the pioneer days. This is a super fun craft to do with your kids. You could squeeze the juice from the berries yourself if you want, but I do the cheaper and easier way and just buy juice concentrate... grape, cherry and cranberry all make very pretty ink.
You can use feathers, pipe cleaners or paint brushes to write with your berry ink. Paint brushes are the easiest, but feathers are the most authentic. This is a great pioneer craft for a pioneer family home evening or anytime!