Writing with Rock with Blackberry Juice I wanted to write in my own blood. I know where to find blood and I am familiar with its properties as a type of ink, thanks to one rather misguided seventh-grader. Unfortunately, the assignment forbade my first instinct and forced me to get a little more creative. Growing up, I was forced to pick berries with my parents every summer. All of my summer memories include stained, sticky fingers. The idea of using berries as ink was my next idea.