Dark blue is the only Monopoly street color group with two properties.
In the classic United States board, the dark-blue group contains Park Place and Boardwalk. Most other street groups contain three properties, while the brown and light-blue groups also have two. The dark-blue group is distinctive because it has the game's highest purchase prices and rent values.
Park Place costs $350 and Boardwalk costs $400. A player owning both receives the color-group monopoly and can build houses and hotels, subject to the even-building rule and the limited supply of buildings. Boardwalk's hotel rent reaches $2,000, the highest printed rent on a standard property.
The wording matters because dark blue is not the only two-property group: brown and light blue also contain two streets. Dark blue is the only two-property group whose two properties are the most expensive streets, and it is often remembered as a pair rather than as a larger three-property set.