Archbishop of Naples Crescenzio Sepe showed the glass vial to the congregation in the city's cathedral and declared: "The blood has half liquified, which shows that Saint Januarius loves our pope and Naples."
Francis, known for his plain speaking, quipped that he and his fellow visitors to the city's cathedral had failed to win the saint's full affection.
"The bishop just announced that the blood half-liquified. We can see the saint only half loves us."
"We must all spread the word, so that he loves us more!" he added.
Each year thousands of Roman Catholic faithful go to the three special services at Naples Cathedral where the dried blood of the fourth-century martyr is said to turn to liquid.
Some people say the blood even increases in mass during these displays, though the church does not officially recognise any of the relic's alleged transformations as a miracle.
The Roman bishop was decapitated during the persecution of Christians during the reign of the emperor Diocletian in 305 AD.
The showing of the vial is eagerly awaited because, according to tradition, whenever the blood has failed to liquify a catastrophe has occurred.
The ceremony has been going on, sometimes up to 18 times a year, for the past six centuries.
In 1527 and 1528 non-liquefaction was followed by the plague. In 1559 famine came and in 1833 cholera raged through the city. In 1944 during World War II non-liquefaction was proceeded by bombing raids by Allied aircraft.
The "half-miracle" came during Francis's one-day visit to the city in which he was greeted by hundreds of thousands of people waving Vatican flags.
Further strengthening his down-to-earth reputation, the pope headed deep into mafia territory, visiting jailbirds and the poor in Naples amid heightened security.
The pontiff arrived in the poor, crime-ridden Scampia area of the city in a popemobile and immediately plunged into a crowd of children and young people, two of whom managed to pose for a selfie with the pontiff.
"Corruption stinks, corrupt society stinks," he told residents, adding that "we all have the potential to be corrupt and to slip into criminality".