Black String (블랙스트링) is a South Korean quartet built around the geomungo, the Korean six-string zither, played by its leader Heo Yoon-jeong (허윤정, billed Yoon Jeong Heo on its records). With her are Aram Lee (이아람) on daegeum bamboo flute, yanggeum hammered dulcimer and danso, Jean Oh (오정수) on guitar and electronics, and Min Wang Hwang (황민왕) on janggu hourglass drum, percussion and voice. The Korea Herald and MusicBrainz both date the group to 2011, and the name is the English rendering of the instrument at its centre. The band calls its music "borderless contemporary Korean music."

According to an Arts Desk interview with Heo, the idea of a band came from the producer Myung-guk Kye during a Korea-UK cultural exchange programme. The first lineup mixed Korean and British players and made its debut in London; once back in Korea, Heo built the present four-piece around the geomungo. Kye is credited as executive producer of the group's first album. Heo is the daughter of the theatre director Heo Gyu, went to the Gugak National High School on his suggestion, where she took up the geomungo, and studied geomungo sanjo with Han Gap-deuk. Korean press has described her as a professor of Korean traditional music at Seoul National University, and in February 2023 she received the grand prize at the first Seoul Art Award for the traditional-music piece Akgaakmu-Jeolgyeong. A CultureHub biography describes Oh and Lee as based in Seoul and co-directors of the music programme at the Seoul Institute of the Arts.

Early appearances abroad included Berlin and Santiago de Compostela. In December 2014 it played the Jazz Korea Festival in Berlin on the day after Mosaic Korea, the touring project band Heo led, opened the event, and a Korean report said the audience had never heard rhythms like these. In October 2016 it was an official showcase artist at WOMEX in Santiago de Compostela. That year it also became, by the account of Korean press and festival biographies, the first Asian group signed to the German jazz label ACT.

Mask Dance (ACT 9036-2) was recorded at Brickwall Sound in Seoul on June 30, 2016 and released on October 14, 2016. It has seven pieces, six of them composed by the band, with Heo as producer; the opener, "Seven Beats," runs 11 minutes. Songlines gave it five stars and an Editor's Choice, and in February 2017 it won the Best Performance prize in the Jazz & Crossover field at the 14th Korean Music Awards. In October 2018 it also won the Asia and Pacific category at the first Songlines Music Awards in London.

Karma (ACT 9045-2) followed on September 27, 2019, with nine tracks. The French-Vietnamese guitarist Nguyên Lê guests on "Elevation of Light" and "Song of the Sea," and the album includes a reworking of Radiohead's "Exit Music (For a Film)" and a closing "Blue Shade." Songlines gave it four stars and an Editor's Choice, DownBeat reviewed it in October 2019, and at the 17th Korean Music Awards in 2020 it won in the Crossover category of the Jazz & Crossover field. The band held its release concerts on November 1 and 2, 2019, and announced European and American tours running through February 2020. On November 18, 2019 it closed the K-Music programme of the London Jazz Festival at the Purcell Room.

Two days after that, on November 20, 2019, Black String appeared at the Berlin Philharmonie in the tenth concert of the Jazz at Berlin Philharmonic series, an evening curated by the ACT founder Siegfried Loch in memory of the producer Nesuhi Ertegun and titled East-West. It shared the bill with the vocal and cello act NES, the Moroccan guembri player Majid Bekkas and Nguyên Lê, and the live album Jazz at Berlin Philharmonic X - East-West (ACT 9913-2) came out on May 29, 2020. Black String plays "Hanging Gardens of Babylon" and "Elevation of Light" on it and joins the others for the encore "Bania."

Concerts resumed in 2021 with a K-Music Festival date at Grand Junction in London on October 28, marking the group's tenth anniversary, with Nguyên Lê as guest; the Arts Desk review called it storm-force in intensity and compared the band's drive to 1970s Miles Davis. In September 2022 it was booked at New York's Chelsea Table + Stage and the Creative Alliance in Baltimore, and on October 30, 2024 it was booked for the K-Music Festival at Kings Place in London, where new material was billed. The third album, Road of Oasis, was released on December 20, 2024 by Mirrorball Music, with seven tracks including "Reveal," "East Sea" and "The Abyss." In July 2026 the Korea Times named Black String among the acts planned for the second half of Korea's guest-country programme at Jazz Fest Budapest 2026.

The instrumentation is the identity. The geomungo supplies melody and deep percussive weight, the daegeum and danso carry breathy lines, the yanggeum adds a struck-string shimmer, the janggu drives the rhythm, and Oh's electric guitar and electronics bring in rock and electronic textures. Simon Broughton of Songlines found Mask Dance distinctly Korean despite the Western guitar and called the band one of South Korea's most exciting. Robin Denselow, reviewing Karma in the same magazine, grouped Black String with other Korean bands that mix traditional instruments with guitars and electronics, and noted that the record moves like a jazz set and ends with a piece that draws on Ornette Coleman.

How far that makes it a jazz group is a fair question, and the evidence points both ways. Its albums were released by ACT, a jazz label; Discogs files them under Jazz and Folk, World and Country, with free improvisation and avant-garde jazz among the style tags; Apple Music lists Mask Dance and Karma as jazz; and both Korean Music Awards were given in the Jazz & Crossover field. Against that, Heo has described the music as free music that goes beyond boundaries rather than jazz, though she says it shares jazz's improvisation, and DownBeat's reviewer heard jazz as one of several influences folded into a folk-based sound. Jazzwise, reviewing the Berlin live album, framed it as a test of how much world music a jazz listener will accept. Black String is best placed as a group on the border of jazz and Korean traditional music, with work that belongs to both scenes.