Blackmun was speaking following the latest wave of revelations surrounding hosts Russia at the Sochi 2014 Olympic Games, where they topped the medals table.
According to former Moscow Laboratory head Grigory Rodchenkov, 15 home medal winners were implicated in an intrinsic doping programme in which anabolic steroids were mixed with alcohol, before urine samples were switched in a clandestine night-time operation.
The official, who has left Russia for Los Angeles, said this was so effective it ran "like a Swiss watch" in an interview with the New York Times.
It followed other revelations of systemic and state sponsored doping in Russia, with the country's athletics team facing missing Rio 2016 unless the All-Russia Athletics Federation's suspension by the International Association of Athletics Federations (IAAF) is lifted in time.
"It is increasingly difficult to defend the current system following a breakdown of this magnitude," Blackmun told the Associated Press.
"If the recently reported allegations prove to be true, we need to admit that the system is flawed.
"We need to fix it, and we need to find a way to assure the athletes in Rio that they are competing on a fair and level playing field.
"As a global sporting community, we need to embrace the opportunity to shine a light on the bad actors who are responsible for the wrongdoing and corruption.
"We are at a defining moment for international sport.
"It is time for strong leadership and decisive action.
"Doping is a problem all around the world, not just in Russia."
In admitting that sport faces a "defining moment", Blackmun's words are stronger than those of International Olympic Committee (IOC) President Thomas Bach, who has repeatedly insisted to be adopting a "zero tolerance" approach but is not drawing firmer conclusions until an investigation into Rodchenkov's allegations is concluded.
It comes after suggestions that an investigation will be launched by the American Department of Justice into the Sochi 2014 claims that is separate from those being carried out by sporting bodies.
This call has only been made by one US attorney prosecutor, and could be rendered obsolete by personnel changes following the Presidential Election later this year.
But it does indicate a fresh desire to make firmer gestures on doping by a high-level authority in the US - possibly the administration of soon-to-depart President Barack Obama - with this also potentially behind the words of Blackmun, an administrator who is usually more restrained with his public messages.
Russia has dismissed all American and Western criticism, claiming it is politically motivated.
An article was published in Sputnik today entitled: "US Plays 'World’s Prosecutor' in Doping Probe Against Russian Athletes".
Numerous other countries are also facing major problems, including Kenya, which was declared non-compliant with the World Anti-Doping Agency (WADA) last week, and Morocco, where six arrests have reportedly been made in a long-running investigation into doping within the country's athletics Federation.
The United States has also faced problems, with seven Major League Baseball (MLB) players having failed tests since the latest season began just six weeks ago.
Some of the world's most notorious doping cheats of recent years - including cyclist Lance Armstrong and sprinter Justin Gatlin - also hail from the nation.
Similarly strong sentiments about the state of sport have been uttered this week by Richard Pound, the senior IOC member from Canada who chaired the initial WADA Independent Commission investigation into Russian athletics.
"This will be a major credibility issue for the IOC and, indeed, for the Olympic Movement as a whole," he wrote in an editorial published in the Globe and Mail.
"Will the philosophy of fair play, honesty and the protection of clean [non-doping] athletes prevail?
"Or will those principles be sacrificed on some political altar?
"Do the rules of sport apply to major countries, or only to smaller countries?
"Does integrity matter?
"We are at a tipping point for international sport, whose future may well be in doubt."