This is a loaded question like "when did you stop beating your wife?", as it takes for granted a counterfactual premise, specifically that Benedict XVI questioned the historicity of the crowd saying, "Let his blood be upon us and upon our children". But B_XVI did not question the historicity of the crowd gathered before Pilate saying that, but the historicity of the literal understanding of the description of that crowd by Matthew, namely "all the people" (pas ho laos), as comprising all the Jews living at that time or even all the Jews in Jerusalem at that time.
In Mark's Gospel, the circle of accusers is broadened in the context
of the Passover amnesty (Barabbas or Jesus): the "ochlos" enters the
scene and opts for the release of Barabbas. "Ochlos" in the first
instance simply means a crowd of people, the "masses". The word
frequently has a pejorative connotation, meaning "mob". In any event,
it does not refer to the Jewish people as such. In the case of the
Passover amnesty (which admittedly is not attested in other sources,
but even so need not be doubted), the people, as so often with such
amnesties, have a right to put forward a proposal, expressed by way of
"acclamation". Popular acclamation in this case has juridical
character (cf. Pesch, Markusevangelium II, p. 466). Effectively this
"crowd" is made up of the followers of Barabbas who have been
mobilized to secure the amnesty for him: as a rebel against Roman
power he could naturally count on a good number of supporters. So the
Barabbas party, the "crowd", was conspicuous, while the followers of
Jesus remained hidden out of fear; this meant that the vox populi, on
which Roman law was built, was represented one-sidedly. In Mark's
account, then, in addition to "the Jews", that is to say the dominant
priestly circle, the ochlos comes into play, the circle of Barabbas'
supporters, but not the Jewish people as such.
An extension of Mark's ochlos, with fateful consequences, is found in
Matthew's account (27:25), which speaks of "all the people" and
attributes to them the demand for Jesus' crucifixion. Matthew is
certainly not recounting historical fact here: How could the whole
people have been present at this moment to clamor for Jesus' death?
It seems obvious that the historical reality is correctly described in
John's account and in Mark's. The real group of accusers are the
current Temple authorities, joined in the context of the Passover
amnesty by the "crowd" of Barabbas' supporters.
We know from John's Gospel that the crowd had gathered at the stone-paved square in front of the praetorium ("Lithostrōton") (Jn 19:13). Is it just reasonable to think that all adult Jews at that time, or even all adult inhabitants of Jerusalem, could fit in that square? It is clear that what Matthew means by "all the people" is "all the people in that place at that moment", not "all the people of Jerusalem" and even less "all the Jewish people".