Calendula is actually a real substance: it’s a herb derived from the plant of that name. The plant is a small yellow flower that grows across Mediterranean Europe and the Middle East, from the Azores to Iran.
It actually has medicinal effects – it has traditionally been used to treat constipation, abdominal cramps, acne, and in modern eras, radiation dermatitis and tumours. It is also used in cooking.
What lands it on this list is its supposed magical power: it is used by some members of the staff at Wolfram and Hart as a means of cheating psychic scans, by allowing users to hide their true thoughts and feelings.