Regarding casual conversation

"It's Jack's fault."

Izzy's example is good, but if you want to use the word "blame" in this situation (at a party), "Jack is to blame" is too formal and legalistic. Mary would more likely point her finger and say, "Blame it on Jack" or just, "Blame Jack."

Then Mary pointedly spat, 'I'm jack of Jack's jackshit and Jack's to blame'.

“It is Jack to blame” isn’t really how a native speaker would say it. The natural version is “Jack is to blame,” and in quick speech it often gets shortened to “Jack’s to blame.”

“Jack to blame” by itself sounds more like a newspaper headline than something someone would say out loud, so Mary probably wouldn’t use it in conversation.

If she’s pointing at him at the party, I agree with the others that something shorter and more casual fits better. Things like “Blame Jack,” “It was Jack,” “That’s on Jack,” or even just “Him!” while pointing would sound normal.