The San Diego Trolley Blue Line is the first line of the San Diego Trolley system that started the modern 'Light Rail System' craze in the United States when what was known as the South Line opened from Santa Fe Depot in downtown to San Ysidro/International Border on July 26, 1981 along a sparsely used freight ROW (which still sees minimal freight service). The terminus in San Ysidro is just steps from Mexico and the busiest land border in the world. Originally the line opened with just 20 minute headways, on the single track railway with timed meets at four passing sidings and ground-level platformed station’s built cheaply. Strong ridership led to the double-tracking of the original line by February 1983, and the purchasing of additional trolley cars for more frequent service.

The Blue Line wasn’t extended again until the 1990s, first in July 1992 one stop to County Center/Little Italy, and then on June 1, 1996 to Old Town. It was finally extended to Mission San Diego, on the Mission Valley Line on November 23, 1997.

For the first two decades of the twentieth-century the Blue Line actually got shorter, with its northern portion absorbed into the new Green Line. The first phase pushed the Blue Line’s northern terminus back to Old Town, where trains connected to the new Green Line (limited rush hour service still ran to Qualcomm Stadium for the first couple of years) for service on the Mission Valley Line on July 8, 2005. In September 2012, the first phase of the Trolley Renewal Project, that rebuilt the northern Blue Line station platforms from being four inches resulted in service being pushed back to America Plaza in Downtown with the Green Line extended into Downtown taking over this former Blue Line service.

The Blue Line originally used Siemens–Duewag U2 low-floor trolley vehicles, the only North American Operator of these trolley vehicles (the two Canadian operators, Edmonton which opened in 1978 and Calgary in May 1981 use the high-floor variant) From 2012 through 2015 all of the remaining Blue Line stations were completely rebuilt as part of the Trolley Renewal Project. All stations were rebuilt to have 6 inch platforms instead of ground-level boarding, plus modern canopies and other station improvements along the line south to San Ysidro. This allowed modern Low-Floor S70 Ultra Short trolleys to operate on the Blue Line starting in January 2015, and all U2 trolley vehicles were retired from service the same day. One still runs around downtown on occasion as the Silver Line heritage trolley (which also has a fleet of two PCCs).

The Trolley Renewal Project improved the passenger experience for everyone. ADA riders can now board fully unassisted by pushing a button themselves at any door (the closest of the two doors to the articulating middle section of each trolley) with a bridge plate-like ramp automatically deployed and no longer forced to only board the front door of the frontmost trolley with the help of the driver to deploy the former manual wheelchair lift. ADA riders were given agency to decide which trolley car they want to ride in, and not be forced to ride in the same space exclusively with other ADA riders. Delays on the Blue Line were also reduced because trains were no longer delayed for the loading and unloading of passengers needing to use the slow lift (identical to those used on high-floor buses) that required longer station stops.

Blue Line service was finally extended north again on November 21, 2021 on the fully grade-separated Mid-Coast Extension from Old Town to UTC Transit Center, with service restored, now sharing the tracks with the Green Line between Old Town and Santa Fe Depot.

For service, Blue Line service runs every 15 minutes throughout the day from early morning through late evening. During weekdays, when the core of the line operates with a 7.5 minute frequency, only half of all Blue Line trains run to UTC (still providing 15 minute service on the new line), with the rest short-turning at America Plaza. This same service pattern operated in 2012 and before with only 15 minute base service between America Plaza and Old Town, and during rush hours with trolleys short-turning at America Plaza provided 7.5 minute service on the trolleys busiest branch between Downtown San Diego and San Ysidro/International border.