Interestingly, the other crystal I use in my readings is one that answers to the base chakra, and with its gold sheen, the solar plexus chakra. Now traditionally, black stones are the very antithesis of the third eye and crown chakra stones in that they promote grounding primarily, which from what I've read may hinder the higher chakra's psychic propensities.
Black obsidian is a very different animal tho. As far as I know, when polished and crafted into mirrors, it has been used in scrying by some Mesoamerican cultures, like the Toltecs and the Mayans. To the Aztecs, obsidian was a divine stone, thought to be a form of blood originating from deep in the earth. It was sacred to the god Tezcatlipoca ("Smoking Mirror"), who presided over magic, divination, the night sky, and ancestral memory. The celebrated Elizabethan sorcerer John Dee was said to have acquired a black obsidian mirror from the Aztecs which is now displayed in the British Museum. As one account stated:
. . . an Obsidian Mirror was used by John Dee (1527-1608), a famous astrologer, mathematician, magician and advisor to Queen Elizabeth I. The mirror had been brought to Europe by Cortez, after his conquest of Mexico between 1527 and 1530. John Dee and his associate Edward Kelly used the mirror to ‘see’ the Angel’s oracle text, messages from angels that were pertinent to political and social events of the Elizabethan Court and highly valued by this society.
Also worthy of note are some accounts that say that one of the most well known and famous prophets, Nostradamus, is said to have used a black obsidian mirror as well as a dish filled with ink to make his famed predictions.