Yokohama Municipal Subway Line 1 (Blue Line)

The line grew out of the "Yokohama Six Major Projects" framework announced in October 1965, which positioned a subway as the backbone of the city's transport. On 15 July 1966 the Urban Transport Council's Report No. 9 recommended four subway lines (Lines 1–4) for the city to build, and Yokohama City Council approved the construction plan on 11 October 1966. Lines 1 and 3 were planned to meet and exchange trains at Kannai, which was laid out as the junction of the priority-construction sections; the formal starting point of Line 1 is Kannai, although the operating origin of through services is Shōnandai. The line fitted automatic ticket gates at every station from its opening, a rarity in the Tokyo region at the time.

Construction began with a groundbreaking ceremony on 1 October 1968, and the first trains ran on 16 December 1972, when the 5.2 km first-phase section of Line 1 opened between Isezakichōjamachi and Kami-Ōoka with three-car 1000 series sets. The opening had slipped repeatedly: a July 1970 cloudburst flooded about 3 km of the partly built Kami-Ōoka–Yoshinochō tunnels with some 500,000 tonnes of water and silt, taking about two months to clear, and a September 1972 typhoon then flooded the nearly finished platform at Isezakichōjamachi, pushing the start from a planned spring date to December.

On 4 September 1976 the second phase opened: Line 1 was extended 2.8 km southwest from Kami-Ōoka to Kaminagaya, and the first section of Line 3 opened from Isezakichōjamachi through Kannai to Yokohama. From that day Line 1 and Line 3 began running through one another as a single service across Kannai, the operating pattern that still defines the line. With the completion of the Kaminagaya depot, trains were lengthened from three to five cars on 15 June 1977, and on 21 June 1984 the air-conditioned 2000 series entered service and six-car operation began.

The line then pushed south and west toward Totsuka. On 14 March 1985 the third phase extended Line 1 the 2.0 km from Kaminagaya to Maioka, opened as a single track, on the same day that Line 3 reached Shin-Yokohama. Line 1 reached Totsuka on 24 May 1987 as the fourth phase, though Totsuka opened only as a temporary station because work beneath the Japanese National Railways lines was running late; the permanent Totsuka station opened on 27 August 1989.

Line 1 was completed in 1999. On 29 August that year the sixth and final phase carried the line from Totsuka to Shōnandai, bringing Line 1 to its full 19.7 km and the through-operated Blue Line to its present 40.4 km; on the same day the intermediate "Shin-Yokohama-Kita" station on Line 3 was renamed Kita-Shin-Yokohama to avoid confusion with Shin-Yokohama. Station numbering using the prefix "B" was introduced in 2002, the line's thirtieth year, alongside a 2002 FIFA World Cup tie-in that decorated each of the 32 stations of the through line for one participating nation.

The Transportation Bureau settled on the combined name "Blue Line" for Lines 1 and 3 on 15 June 2006, and began using it formally on 30 March 2008, when the new Green Line opened and the two subway lines needed to be told apart. The veteran 1000 and 2000 series both ended service on 16 December 2006; automatic train operation was switched on in January 2007, platform doors were fitted at every station, and driver-only (one-man) operation began on 15 December 2007. Limited-stop "Rapid" services started on 18 July 2015, cutting the Shōnandai–Azamino run from 69 minutes by local train to 60 minutes. The fleet is now formed of 3000 series and, from 2 May 2022, 4000 series sets, and the city is planning a further 6.5 km extension of Line 3 from Azamino to Shin-Yurigaoka.

• 196615 July: Urban Transport Council Report No. 9 recommends four subway lines (Lines 1–4) for Yokohama to build; the city council approves the construction plan on 11 October.
• 19681 October: the groundbreaking ceremony for the construction of Lines 1 and 3 is held.
• 197216 December: the 5.2 km first-phase section of Line 1, Isezakichōjamachi–Kami-Ōoka, opens; three-car 1000 series sets begin revenue service.
• 19764 September: second-phase opening — Line 1 extended 2.8 km from Kami-Ōoka to Kaminagaya, and the first Line 3 section (Isezakichōjamachi–Kannai–Yokohama) opens; through operation between Line 1 and Line 3 begins at Kannai.
• 197715 June: with the Kaminagaya depot completed, trains are lengthened from three to five cars.
• 198421 June: the 2000 series, the line's air-conditioned trains, enters service; six-car operation begins the same day.
• 198514 March: third-phase opening — Line 1 extended 2.0 km from Kaminagaya to Maioka (as a single track), the same day Line 3 reached Shin-Yokohama.
• 198724 May: fourth-phase opening — Line 1 extended Maioka–Totsuka, with Totsuka opening as a temporary station because work beneath the JNR lines was delayed.
• 198927 August: the permanent Totsuka station opens, replacing the 1987 temporary facility.
• 199929 August: sixth and final phase — Line 1 extended Totsuka–Shōnandai, completing the 19.7 km Line 1 and bringing the through line to 40.4 km; 'Shin-Yokohama-Kita' on Line 3 is renamed Kita-Shin-Yokohama.
• 2002Station numbering using the prefix 'B' is introduced; for the line's thirtieth year, each of the through line's 32 stations is decorated for one nation in a 2002 FIFA World Cup tie-in.
• 200615 June: the Transportation Bureau decides on the combined name 'Blue Line' for Lines 1 and 3; the 1000 and 2000 series both end service on 16 December.
• 200715 December: driver-only (one-man) operation begins, after automatic train operation was switched on (20 January) and platform doors were installed across the line.
• 200830 March: the name 'Blue Line' comes into formal use as the Green Line opens and the two subway lines need to be distinguished.
• 201518 July: limited-stop 'Rapid' services begin, cutting the Shōnandai–Azamino run from 69 minutes by local train to 60 minutes.
• 20222 May: the 4000 series enters service, supplementing the 3000 series fleet.