The earliest source we have for it is the gospel of Mark, which was probably written around the year 70. In Mark's version of the crucifixion scene, Pontius Pilate the Roman governor doesn't really want to crucify Jesus, but the Jews keep shouting "Crucify him," so he eventually does.

Over the next 20 years or so, we can tell that the desire to blame Jews as a group for the crucifixion only grew because the gospel of Matthew, written around the year 90, has the same interaction except that at the end, the Jews say "His blood is on us and our children!"

So definitely by the year 90, there was a significant part of early Christianity that believed that not only were the Jews who were in Jerusalem when Jesus was crucified responsible, but all Jews, even in generations to come, were responsible.

Reza Aslan's Book Zealot does a great job explaining this in depth, but basically it started around 60-70 ce with the large scale Jewish revolt in Jerusalem that lead to the Romans decimating the city and destroying the temple. Up until that time Christianity was a sect of Judaism still adhering to the Law of Moses. After the revolt the Christians wanted to differentiate themselves from the Jews in order to stay in the good graces of Rome.