I absolutely adore Black Tulasi and Hindi-variant of "Shree Agni-Purana" (which I occasionally read this autumn) mentions not only Green Tulasi, but the Black One as well!!!
You can find Black Tulasi in many Moscow supermarkets under "Basil" (not Tulasi) Name and getting back to "Shree Agni-Purana", the Black Tulasi is mentioned in "Shree Vishnu-Puja" Chapter.
This chapter is very laconic, here it is, just on one screenshot:
