What is Black Rents?

Legal Definition
In old English law. Rents reserved in work, grain, provisions or baser money, in contradistinction to those which were reserved in white money or silver, which were termed "white rents," (reditus albi,) or blanch farms. Tomlins; Whishaw.
-- Black's Law Dictionary
Legal Definition
Rent payable in produce.
-- Ballentine's Law Dictionary