Blalock was born in Dallas, Texas. He attended Plano East Senior High School in the Dallas suburb of Plano. He was three-year starter at offensive guard, but also saw playing time at defensive tackle. As a sophomore in 1999, Blalock was named second-team all-district after recording 151 pancake blocks, as he helped lead Plano East to an 11-3 record and the Texas 5A Division II state quarterfinals that season. The offensive lineman was named all-district and all-area as a junior after registering 130 knockdowns. In 2001, Blalock posted 134 knockdown blocks, did not allow a sack and opened holes for an offense that averaged nearly 300 rushing yards per game. His top game of season was a 21-pancake performance vs. Plano Senior High School.