why does the narrator refer to black hole of calcutta?

The narrator refers to Ranga's courtyard as the Black Hole of Calcutta because the teeming masses had congested that small space into a similar sort of structure where more and more people seemed to be swallowed up as they went in. The Black Hole of Calcutta was a dungeon in the old Fort of William of Calcutta where British prisoners of war were held captive by the nawab of Bengal, Siraj-ud-Daulah. The inhabitants had to live in close quarters, crammed together in a small space. The situation was similar in Ranga's courtyard which had witnessed a huge turnout due to the arrival of the accountant's son from the city.