Not a blackboard, but just a slate. Paper was too expensive for him, so he used it only to write down results.
Source: https://www.quantamagazine.org/three-puzzles-inspired-by-ram...
That’s very fascinating. I guess it still is the case for many people in the world, that paper is too expensive. Whereas for many of us it’s readily available in massive, one might even say infinite, quantities.
An actual, literal case (instead of a cop-out) of "a truly marvelous proof, which this margin is too narrow to contain."
So he did his derivations and proofs on the blackboard, and just wrote down the result in his notebook and then erased the blackboard.
He knew he didn't have room for anything else in his notebooks.