It is unclear when the "circular" Columbia Square, which Charles Bulfinch first laid out in 1801, was changed to its present square form. It may have occurred in 1849, when the City of Boston installed fencing at the perimeter of the squares. This original perimeter fencing can still be seen at the Church of the Immaculate Conception on Harrison Avenue.

In 1850-51 the City installed sidewalks around the two squares, fencing around the fountains, replaced dead trees and repaired the iron fences. On an 1852 City of Boston map, the layout of the two squares with diagonal paths, central fountains and semi-circular lawn panels edged with trees was visible.

In 1913 the Olmstead brothers developed improvement plans for both squares. The plans maintained the original diagonal paths, but added two smaller walks to the squares. On Blackstone Square a walk was added mid-block on Shawmut Avenue and on Franklin Square a walk was added on the East Newton Street side of the square. The 12' wide walks, which were initially crushed stone, were paved with asphalt as part of the proposed park improvements. A 14' wide brick walk was also added around the squares. It is assumed that the initial perimeter fencing was removed prior to 1900, since later photos show low stakes at the perimeter of the squares. The 1913 plans also indicated the original locations of the elms within the squares. The plan for Franklin Square indicated that there were originally 16 trees planted 20 feet apart around the curve of each of the four semi-circles, with approximately 10 trees long each straight edge. Each of the 10 trees were planted about 20 feet from the edge of the square and spaced roughly 20 to 25 feet apart. Included in the 1913 improvements were an additional 13 elms spaced 30' apart on each side of the square. Elms are no longer in the squares; lindens are now the dominant tree species and may have been planted after the 1938 hurricane. The original plan also had a five-foot wide flowerbed surrounding each fountain. Electric lights were located in the center area at the end of the two diagonal walks.

Additional improvements were done by the City in1970. The improvements included new light fixtures throughout the park. The lights consisted of white globes mounted on concrete poles. Base drawings noted that three or four benches were on each of the straight walk sections with a total of 27 benches in Blackstone Square and 33 benches in Franklin Square. A play area consisting of a swing set and climber was also noted in the north east comer of Blackstone Square. The play area was surrounded by a five foot chain-link fence.

In 1980 and 1983 new perimeter fences were installed at each square. The fence exists today, and consists of a simple picket with "tulip" shaped finals. The fountains were refurbished and existing benches that were in poor shape were replaced with benches that were Victorian in appearance.

The most recent improvements occurred in 1995 when the light fixtures were replaced with the traditional City of Boston acorn fixture. Acorn fixtures were also added along the streets and floodlights were added to the fountain.

Throughout numerous renovations, both Blackstone Square and Franklin Square have retained their 1850's appearance of diagonal walks, central fountains, trees and lawn.