The envelope, addressed to an inmate, was discovered Monday during routine mail screening at the Federal Correctional Institution Englewood. Prison spokesman John Sell said a hazardous materials team found that the substance wasn't dangerous.
He wouldn't say which inmate was the intended recipient.
Inmates weren't able to move around the prison while a hazardous materials team checked out the substance.
Sell said local and federal law enforcement authorities were also notified.
Blagojevich reported to the prison March 15 to begin serving a 14-year sentence.