Infill is what planners call new buildings that are built in existing neighborhoods. Infill is when we go back to the random dirt lots that got left behind, and add in buildings that make the neighborhood more complete.
Fresno has grown so fast that vacant lots are scattered throughout the city, even on busy streets like Blackstone. It also grew faster than it could keep up with putting in streetlight and sidewalks, even on busy streets like Blackstone, until they started to be required by the city as a condition of development. Infill benefits everyone in existing neighborhoods, by bringing new investment into older neighborhoods.