The 86-year-old Blanton had asked the Alabama Board of Pardons and Paroles to let him die as a free man, but the panel denied his request Wednesday morning. He won't be eligible again until 2021, the panel decided.
Blanton was convicted in 2001 of murdering Addie Mae Collins, Cynthia Wesley, Carole Robertson, all 14, and Denise McNair, 11, and sentenced to life in prison.
The powerful bomb was planted under the church steps and it collapsed a basement wall into a lounge the four girls used for changing into their choir robes.
The former U.S. attorney who convicted him, Doug Jones, opposed the parole request.
"Fact of the matter is he bombed a house of God on Sunday morning and killed four children and needs to do the time for his crimes," Jones said.
In 2008, during an interview on NPR, Chris McNair recalled seeing his daughter's body on the day that shocked the nation.
"We drove over to a hospital, and we fumbled around, and we found somebody else who had been in the morgue," he told reporter Michele Norris. "And there lay all four of them, there side by side on the table. And Denise was lying out there with a piece of mortar, it looked like a rock, mashed in her head."
Justice was delayed not because the case was a bona fide mystery.
Within days, the FBI suspected that Blanton and his KKK cohorts Robert Edward Chamblis, Bobby Frank Cherry and Herman Frank Cash were responsible for the attack after civil rights protests and other bombings had rocked the city nicknamed "Bombingham."
Alabama Attorney General Bill Baxley convicted the then-73-year-old Chambliss in 1977, after reopening the case in 1971, and then the case remained dormant for decades.
Baxley contended that the FBI, which had informants in the Klan in the 1960s and wiretaps on klansmen, wouldn't share the information that would allow him to build a case against Blanton, Cherry and Cash.
Then, in 1997, after other successful prosecutions for Civil Rights Era murders, the FBI Birmingham office reopened the case. Jones, the U.S. attorney for the Northern District of Alabama during the Clinton administration, made it a personal crusade.
Chamblis and Cherry, who was convicted in 2002, died in prison. Cash, who died in 1994, was never charged in the case.
The Klan targeted the church because it was a leading African-American institution and served as a staging spot for the marches that met fierce resistance from city Commissioner of Public Safety Bull Connor, brutality that has lived on in television footage and defines the city for many people to this day.
The Klan hoped the attacks would derail the movement -- the marches had wrung concessions from local leaders and the state had begun integrating schools days before the bombing -- but historians contend the church bombing marked a turning point in the civil rights movement and contributed to the passage of the Civil Rights Act of 1964.
Blanton is held in the St. Clair Correctional Facility near Birmingham, where he is serving four back-to-back life sentences.
Alabama allows a convict to be considered for parole after 10 years or after serving one-third of his sentence, whichever comes first, according to the parole board website.
It is unknown if Blanton had an attorney representing him for Wednesday's parole board hearing.
Blanton has never shown remorse or accepted responsibility for killing children, which alone should have disqualified him for parole, Jones said.
"Would we let a terrorist out after 15 years? I don't think so," he said before the panel's decision. "That's the issue. The law says he gets punished for it. That's what should happen."