It is 25.5 kilometres (15.8 miles) long, and runs from Kungsträdgården via T-Centralen to Västra skogen where it branches in two, and continues to Hjulsta and Akalla as lines 10 and 11 respectively.

On 5 June 1977, the branch from Hallonbergen to Akalla opened, and on 30 October the same year, the line was extended in the opposite direction from T-Centralen to Kungsträdgården.

The primary reason for constructing the line was the need for public transport to serve the newly-built large residential areas in the northwestern part of Stockholm, built during the period 1965–1975.

However, in 2013, it was decided that the line will run from Kungsträdgården to Nacka centrum via Sofia, Hammarby canal, Sickla and Järla.

The route through the Slakthus area was finalised in May 2015, along with a new underground station to replace Globen and Enskede gård on the Green line.

Although the Blue line is almost entirely in tunnel, it also has the metro's longest elevated section at Kista, 1,200 metres (3,900 ft) long.