From the state line, the route heads southeast on two-lane undivided Blackhawk Road, passing through forested areas with a few fields.

Farther east, residential development near the road increases and it enters Chippewa Township, heading east-northeast.

In this area, the route passes to the south of Beaver County Airport before it makes a turn southeast onto Darlington Road.

From this point, PA 251 becomes 37th Street Extension and passes more residences, entering the borough of West Mayfield.

Upon passing over Norfolk Southern's Fort Wayne Line, the route crosses into the city of Beaver Falls and becomes 37th Street, heading through residential and business areas before ending at PA 18.