ksho said:
This is even more likely when you consider the conversation Medea had with EMIYA after he defected in UBW. She wondered why evil spirits like herself could be summoned by the grail as a servant, as this wasn't possible in the past. I don't quite remember EMIYA's reply...but he said something along the lines about the grail being tainted during the 3rd/4th war?
That's where i speculated that the taint in question was Angra Mainyu, and it is thanks to his influence that evil spirits like medea could be summoned into the seven original classes.
That was referring to how Angra Mainyu itself was left behind in the Grail after the 3rd Grail War.
Remember, when a Servant dies, the soul of the Eirei and all the power it possesses flows back into the Grail. Avenger was a "hack" in the Grail system by the Einzberns, who intended (and succeeded) to summon Angra Mainyu... but AM turned out to be a weak Servant and died. However, with AM going into the Grail (when formerly, there were protections in place against "villainous" or "monstrous" Heroic Spirits being summoned), it essentially broke the safeguards and tainted the Grail.
The presence of Angra Manyu tainting the Grail is why that "cursed black mud" was pouring out of the Grail. It's also what gives Sakura her connection to AM itself, since Zoken specifically used broken shards from the tainted Grail in the creation of Sakura's worms.