Blanton, along with three other Klansmen, had long been suspected of involvement in the bombing, but prosecutors say they never had enough evidence to bring him to trial.
Two years after the bombing, the FBI concluded four men were involved, but authorities closed the case in 1968 without filing any charges.
State prosecutors reopened the case in the late 1970s and one man, Robert "Dynamite Bob" Chambliss, was brought to trial and convicted on state murder charges in 1977. Chambliss, who denied any role in the bombing, died in prison in 1985 while serving a life sentence.
The case was reopened again in 1980 and 1988 but resulted in no additional charges. At the urging of black ministers, the FBI reopened the case in 1997.
Of the four men originally suspected, Chambliss and Herman Cash are dead. Along with Blanton, Bobby Frank Cherry was indicted last year. Among the evidence that helped prosecutors win indictments was the testimony of Cherry's granddaughter, Teresa Stacy, who said she overheard her grandfather talking about the bombing.
Cherry's trial has been delayed while psychiatrists determine whether the 71-year-old is mentally competent to stand trial.
ABC affiliate WIVT in Birmingham contributed to this report.