He left the Department in 1967. He then became one of the founders and editors of the Washington Free Press, the first "alternative" newspaper in the capital. In 1969, he wrote and published an exposé of the CIA in which was revealed the names and addresses of more than 200 employees of the Agency. He has worked as freelance journalist in the United States, Europe and South America. From 1972 to 1973 Blum worked as a journalist in Chile, where he reported on the Allende government's "socialist experiment". In the mid-1970s, he worked in London with ex-CIA agent Philip Agee and his associates "on their project of exposing CIA personnel and their misdeeds". He supports himself with his writing and speaking engagements on college campuses.

In his writing, Blum devotes substantial attention to CIA interventions and assassination plots. He has supported Ralph Nader's presidential campaigns. He currently circulates a monthly newsletter by email called "The Anti-Empire Report".

In early 2006, Blum briefly became the subject of widespread media attention when Osama bin Laden issued a public statement in which he quoted Blum and recommended that all Americans read A Guide to the World's Only Superpower. As a result of the mention sales of his book skyrocketed. Rogue State shot up from 205,763 to 26 on Amazon.com's index of the most-ordered books. In a May 22, 2006 article entitled, Come Out of the White House With Your Hands Up, Blum wrote, "Since the bin Laden recommendation, January 19, I have not been offered a single speaking engagement on any campus . . .This despite January-May normally being the most active period for me and other campus speakers."