It’s “the only one so far,” says Kailash Sahu, an astronomer at the Space Telescope Science Institute in Baltimore.

In 2022, Sahu and his colleagues discovered the dark object coursing through the constellation Sagittarius. A second team disputed the claim, saying the body might instead be a neutron star. New observations from the Hubble Space Telescope now confirm that the object’s mass is so large that it must be a black hole, Sahu’s team reports in the April 20 Astrophysical Journal.

While solitary black holes should be common, they are hard to find. The one in Sagittarius revealed itself when it passed in front of a dim background star, magnifying the star’s light and slowly shifting its position due to the black hole’s gravity.

As far as I know, the first use of the phrase "black hole" in the modern astronomical sense in science fiction is in Rift in Space (1937) by Paul Ernst:

...A huge, full circle of the sky before them was a circle of blackness, with no stars showing. It was as though a great plate were being held up before the T-12.

Only you couldn’t see the plate...

...“All right,” snapped Gates, “soar up here till you run out of gas and crash anyway. For that thing has gravity force. Already we weigh more. There’s enough gravity pull to smash us as thoroughly as if we crashed from a hundred miles up on Earth.”

...The round black hole in the star studded sky was growing rapidly more all-engulfing, indicating that, whatever celestial body it was they approached, it was quite small.
(Read more about the black hole)

However, as the story progresses, it turns out that you can land on it! Not quite the black hole theory we're used to.

To find a black hole in the modern sense, even if the exact name is not used, we go back to 1935 and Star Ship Invincible by Frank K. Kelly. In the story, space explorers encounter a "the Hole - a rift in space". It wanders alone. The story also has an illustration that is probably the first depiction of a "black hole":

THEY went over to the Danler spacial chart and stood staring at it together. A black smear, twisted like a snake, cutting across the smooth ivory luminosity of the board — that was the Sink Hole.

Swinging across the upper end of that ebony blur they could trace the faint red line of the Walton Arc. A green sliver of glow was the Invincible; the ship crawled deliberately, it seemed to them, along the scarlet curve. As it crawled, it came nearer and nearer the blob of unfolding darkness that was the Hole. It seemed as though some one had spilled ink on the white surface of the chart, and the ink was spreading, spreading.

(Danler Spacial Chart from 'Star Ship Invincible' by Frank K. Kelly)

GRAHAM kept thinking of the girl; and not of the girl alone but of all of them on the Invincible; crouching in a little metal bubble that hurtled at inconceivable speed along the Walton Arc, along a plotted curve that passed just beyond the edge of the Hole in Space. Just beyond — maybe. If the Hole didn’t grow. And the Hole was growing.

His throat was bitter and dry. “It’s no good if you’ve been preparing against collision. The Hole isn’t solid, isn’t material or tangible. What the ship’s got to have is power — power to pull away.”

“You think,” Garth said slowly, “the Hole is something like a vortex or a whirlpool? And when a ship comes too near it’s sucked under.”

“Now you’ve got it,” Graham said.