Who popularized the term 'black hole' in 1967?

John Wheeler, who did not coin it - he heard it from an audience member at a lecture and adopted it, having been casting about for something better than gravitationally completely collapsed object. The name is the reason the concept escaped physics into everything else.

Wheeler had a gift for that. He also named the wormhole and the quantum foam, and coined the phrase it from bit for the idea that physical reality might be informational at bottom. He was explicit that naming things was serious work, on the grounds that a phenomenon nobody can conveniently refer to gets thought about less.

The objects themselves were resisted for decades. Schwarzschild produced the solution in 1916 within months of general relativity, and Einstein published a paper in 1939 arguing that such things could not form in nature. Chandrasekhar's calculation showing a star above a certain mass must collapse was ridiculed by Eddington in 1935. It took until the 1960s, and the discovery of quasars and pulsars, for the field to accept that the mathematics was describing something real - and until 2019 for anyone to photograph one.