The white powder was in mail addressed to an inmate at Englewood prison outside Denver. Federal Bureau of Prison officials are not revealing whether the inmate was number 40892-424, the ex-governor of Illinois.
Blagojevich, who is currently the biggest name there, is serving a 14-year prison sentence for public corruption.
Ambulances, fire equipment and hazmat units rushed to the Englewood prison Monday morning after routine mail screening turned up what was described as a powdery white substance in an envelope.
There were no evacuations but for a time prisoner security was stepped up.
Since 9/11, there have been hundreds of white powder occurrences across the country - most of them benign. But because the first instances of white powder showing up in mailrooms turned out to be deadly anthrax, authorities do not take chances.
In 2001, five people were killed by mailed-in anthrax spores and 17 sickened.
The response is especially urgent when white powder is detected at a federal facility.
In Monday's case, the powder was tested and found to be harmless, although authorities have still not released why it was mailed to an inmate.
It was less than three weeks ago that Mr. Blagojevich walked into the prison and took his place behind the wall, surrendering to begin his lengthy sentence.
Just last week, the defrocked governor's one-time chief of staff, John Harris, was sentenced. Because of Harris' considerable cooperation with federal prosecutors, he received just a 10-day sentence.
In a current post on her Facebook page, ex-first lady Patti Blagojevich chides that sentence, writing: "I can't help but wonder what planet we are on. 10 days vs. 5110 days, a sentence that is 51,100% higher than Rod. How do you explain that to your children?"
What Mrs. Blagojevich probably meant was that Harris' sentence was 51-thousand percent less than Rod's, but her Facebook friends got the idea. More than 100 chimed in that the disparity is sad and unfair.
Back to the white powder for a moment: prison inmates have been known to send it to themselves for attention or in some cases to be transferred to another facility. There is no indication that happened in this case though.