Blokhin served in World War I in the Imperial Russian Army. In March 1921 he joined the Cheka and quickly drew the attention of Russian leader Joseph Stalin for his skill at carrying out executions, assassinations and interrogations, as well as torturing and intimidating suspects. Having gained Stalin's favour, Blokhin was quickly promoted and within six years he was head of the Kommandatura Branch of the Administrative Executive Department of the NKVD. During his time in this position, he served under three different NKVD chiefs: Genrikh Yagoda, Nikolai Yezhov and Lavrentiy Beria, two of whom he would later personally execute under Stalin's orders in addition to overseeing all mass executions undertaken by the NKVD.
As senior executioner, Blokhin had the official title of commandant of the internal prison at the Lubyanka, which allowed him to carry out his duties with a minimum of scrutiny and no official paperwork. Although most of the estimated 828,000 NKVD executions conducted in Stalin's lifetime were performed by local Chekists in concert with NKVD troikas, mass executions were overseen by specialist executioners from the Kommandantura.
In addition to overseeing the mass executions, Blokhin personally pulled the trigger in all of the individual high-profile executions conducted in the Soviet Union during his tenure, including those of the Old Bolsheviks convicted at the Moscow Show Trials; Marshal of the Soviet Union Mikhail Tukhachevsky (convicted at a secret trial); and two of the three fallen NKVD Chiefs (Genrikh Yagoda in 1938 and Nikolai Yezhov in 1940) he had once served. He was awarded the Badge of Honor for his service in 1937.
Blokhin's most infamous act was the April 1940 execution by shooting of about 7,000 Polish prisoners interned in the Ostashkov prisoner of war camp in the Katyn forest. The majority were military and police officers who had been captured following the Soviet invasion of Poland in 1939. The event's infamy also stems from the Stalin regime's orchestration of the murders, and the subsequent Soviet propaganda campaign which blamed Nazi Party for the massacres.
In 1990, as part of Glasnost, Mikhail Gorbachev gave the Polish government the files on the massacres at Katyn, Starobelsk and Kalinin (now Tver), revealing Stalin's involvement. Based on the 4 April secret order from Stalin to NKVD Chief Lavrentiy Beria (as well as NKVD Order № 00485, which still applied), the executions were carried out over 28 consecutive nights at the specially constructed basement execution chamber at the NKVD headquarters in Kalinin, and were assigned, by name, directly to Blokhin, making him the official executioner of the NKVD.
Blokhin initially decided on an ambitious quota of 300 executions per night; and engineered an efficient system in which the prisoners were individually led to a small antechamber—which had been painted red and was known as the "Leninist room"—for a brief and cursory positive identification, before being handcuffed and led into the execution room next door. The room was specially designed with padded walls for soundproofing, a sloping concrete floor with a drain and hose, and a log wall for the prisoners to stand against. Blokhin would stand waiting behind the door in his executioner garb: a leather butcher's apron, leather hat, and shoulder-length leather gloves. Then, without a hearing, the reading of a sentence or any other formalities, each prisoner was brought in and restrained by guards while Blokhin shot him once in the base of the skull with a German Walther Model 2 .25 ACP pistol. He had brought a briefcase full of his own Walther pistols, since he did not trust the reliability of the standard-issue Soviet TT-30 for the frequent, heavy use he intended. The use of a German pocket pistol, which was commonly carried by German police and intelligence agents, also provided plausible deniability of the executions if the bodies were discovered later.
An estimated 30 local NKVD agents, guards and drivers were pressed into service to escort prisoners to the basement, confirm identification, then remove the bodies and hose down the blood after each execution. Although some of the executions were carried out by Senior Lieutenant of State Security Andrei Rubanov, Blokhin was the primary executioner and, true to his reputation, liked to work continuously and rapidly without interruption.
In keeping with NKVD policy and the overall "wet" nature of the operation, the executions were conducted at night, starting at dark and continuing until just prior to dawn. The bodies were continuously loaded onto covered flat-bed trucks through a back door in the execution chamber and trucked, twice a night, to Mednoye, where Blokhin had arranged for a bulldozer and two NKVD drivers to dispose of bodies at an unfenced site. Each night, 24–25 trenches were dug, measuring 8 to 10 metres (26 to 33 ft) in length, to hold that night's corpses, and each trench was covered over before dawn.
Blokhin and his team worked without pause for 10 hours each night, with Blokhin executing an average of one prisoner every three minutes. At the end of the night, Blokhin provided vodka to all his men. On 27 April 1940, Blokhin secretly received the Order of the Red Banner and a modest monthly pay premium as a reward from Joseph Stalin for his "skill and organization in the effective carrying out of special tasks". His count of 7,000 shot in 28 days remains the most organized and protracted mass murder by a single individual on record, and saw him being named the Guinness World Record holder for 'Most Prolific Executioner' in 2010
In 1953 Stalin died. Beria, who Blokhin had once served under, forcibly retired Blokhin that March. Following Nikita Khrushchev's rise to power that very year, Blokhin was stripped of all the accolades given to him by Stalin, causing him to sink into alcoholism and depression and eventually driving him to suicide.