Short answer: There is a liquid that does the congeal and liquefy trick, but it's probably not blood.
Long answer: First of all, the Catholic Church has not made an official stance on this event. The wikipedia page on this says so, as well as an investigation by The Italian Committee for the Investigation of the Paranormal.
While the Catholic Church has always supported the celebrations, it has never formulated an official statement on the phenomenon, and maintains a neutral stance about scientific investigations.
AND
The Church, as usual, has sidestepped the issue. It has never claimed that the San Gennaro event is a miracle but it has not allowed independent tests on the phials containing the blood.
The major difficulty in actually examining this substance is that no one will allow a vial to be opened to perform an exam, so only observations through the glass can be made.
In 2005, the Italian Committee for the Investigation of the Paranormal has gone on to look into the issues more.
the red-coloured contents are a thixotropic substance, based on iron chloride. This means that it liquefies when stirred or vibrated and returns to solid form when left to stand. According to Cicap, the substance was probably stumbled upon by an alchemist or a painter in medieval times.
This interest may have been sparked by a 2000 airing on the History Channel about this same phenomenon. Skeptical inquirer made a point to also talk about this. A web archive link is reproduced below (to prevent link rot):
Don't miss "This Week in History" tonight at 9:00 pm (Eastern time US) on
the History Channel. CSICOP's Joe Nickell will appear in a segment on the
Shroud of Turin. Fri, 22 Sep 2000
The Miracle Blood of San Gennaro
Amherst, NY (September 22, 2000)--On Tuesday, September 19, 2000, in Naples,
Italy, an expected "miracle" occurred: a congealed substance many believe is
the blood of a martyred saint liquefied in its vial. San Gennaro (in English
Saint Januarius) is Naples' patron saint, and the superstitious believe that
if the "blood" does not liquefy disaster may strike. In 1980 a massive
earthquake struck southern Italy after one failed liquefaction, yet in 1631,
after the "miracle" had taken place, Vesuvius erupted, killing 18,000.
The glass vial containing the bloody substance is housed in a monstrance (a
shrine in the form of a stand) which is brought out of its vault twice
yearly: on the Saturday preceding the first Sunday in May, and on September
19, the saint's feast day. The latter is the anniversary of San Gennaro's
legendary death.
The Catholic Church has never been able to document Gennaro's existence as
an actual historical personage, and in the 1960s his importance was greatly
diminished, along with the other folkloric figures like Saint George the
legendary dragon slayer. T*here is no historical record for the alleged
martyr's "blood" until the late fourteenth century, more than a millennium
after his reputed beheading*.
The congealed substance itself has never been reliably tested, despite the
local church's claims based on dubious spectral analyses. In fact, the
substance has qualities that differentiate it from genuine blood. Adding to
skeptic's suspicions, there are some twenty other saints' bloods that
liquefy, virtually every one of them from the Naples area--suggestive of some
local secret.
Two skeptical hypotheses have been offered to explain the Januarian
phenomenon by natural means--both suggesting that it is a pious fraud. In
1991 a group of Italian scientists--Luigi Garlaschelli and two colleagues,
Franco Ramaccini and Sergio Della Sala--demonstrated that they could simulate
the effect with a thixotropic gel, a substance that liquefies when agitated
and resolidifies when allowed to stand.
About the same time two American researchers--forensic analyst John F.
Fischer and paranormal investigator Joe Nickell--showed they could reproduce
a liquefying and recongealing miracle blood using a colored oil-and-wax
mixture. When slightly warmed by heat sources such as nearby candles and body
heat, the bogus blood can suddenly liquefy. Nickell and Fischer jokingly
attribute their phenomenon to "Saint Februarius."
Although suitable tests of San Gennaro's alleged blood are still not
permitted, the Italian scientists were allowed to test one of the other
Naples-area vials without breaking it open or damaging its contents. It did
not respond to being shaken, but it did liquefy when warmed gently by a hair
dryer.
For a discussion see Joe Nickell with John F. Fischer, "Mysterious Realms:
Probing Paranormal, Historical, and Forensic Enigmas", Amherst, NY:
Prometheus Books, 1992, pp. 145-164.
As mentioned in this write up, there is an Italian Scientist that has discussed the thixotropic hypothesis in great detail. Sadly, I don't read Italian that well, so I am trusting that this is a faithful translation to English.
Introduction
A sealed glass vial containing a dark unknown substance, allegedly the clotted blood of San Gennaro (St Januarius), is shown several times a year to a packed crowd in the Cathedral of Napoli (Naples). Whilst the container is being handled during a solemn ceremony, the solid mass suddenly liquefies before everybody's eyes. 1
This well-documented phenomenon is still regarded as unexplained 3 by believers and sceptics alike. Noted parapsychologist Hans Bender defined it the paranormal phenomenon with the best and historical documentation; 4 physicist Enrico Fermi seems to have expressed interest as well.
It is also one of the few recurrent non-medical, physical "miracles" that might be studied scientifically.
Read the whole paper, but the conclusion says:
Conclusion
Further tests to investigate the real nature of the holy "blood" without opening the ampoule come readily to mind: for example, molecular absorptions and fluorescence spectroscopy, and Raman scattering measurements, made with modern electronic instruments by qualified spectroscopists. Controlled temperature increments and shock tests also represent non-destructive analytical methods by which our or alternative hypotheses might be verified or disproved. Whether these simple tests will be allowed to go ahead wholly depends upon the Catholic Church. At present however, given that the phenomenon has been replicated, it would be rather too naive to consider it irreproducible or unexplainable.
In the paper, they go into a great deal of detail on not only the thixotropic hypothesis, but also to talk about other rational approaches to the problem. Not only focusing on the thixotropic hypothesis but other possible hoaxes, frauds, or mistakes. The prevailing idea is that this is indeed a fake though, since the chemistry has been known since about the time that this "relic" appeared. This PDF is an extended version of the paper cited above
