Pokeweed is a purplish-red berry found in many parts of North America. In the Midwest and most northern states, it blooms in early fall, typically around mid-September -- just in time for Mabon. The poisonous red berries can be used to provide ink for writing - legend has it that the Declaration of Independence may have been drafted in pokeweed ink, although the final version that sits in the National Archives was done in iron-gall ink. Many letters written by soldiers during the Revolutionary and Civil Wars, because it was something that was readily availably -- pokeweed grows over many parts of the country. According to the Ohio State University, pokeweed berries get their name from a Native American word for blood, owing to the color of the juice. Legend holds that tribal shamans used pokeweed berries to rid the body of evil spirits - probably because ingestion led to copious vomiting and diarrhea.
With a little bit of work, you can make your own pokeweed ink to use in magical workings, particularly those used in banishing spells. The ink seems to be sensitive to sunlight and browns when exposed to UV rays, so if you're going to store it, use a dark-colored bottle or store it in a cabinet out of the light.
Warning: the entire plant is toxic to humans, so don't try to eat them!