Blair County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 122,822. Its county seat is Hollidaysburg, and its largest city is Altoona. The county was created on February 26, 1846, from parts of Huntingdon and Bedford counties. The county is part of the Southwest Pennsylvania region of the state. Blair County comprises the Altoona, PA metropolitan statistical area.