But neither character talks about the noose. I think Mac is just dealing with crazier and crazier situations every minute, he's not letting Blair out no matter what he says or does.

Blair is either lying to get back inside, or genuinely wants back in, Mac just gives him some alcohol and says "Trust in the Lord." To me it's ambiguous but not because of the noose, because Mac is treating him like a human, but still not trusting Blair enough to let him out of jail.

The noose could mean anything really. He could have chickened out, a Thing could have taken him over prior to him completing the suicide, heck he could be a Thing that doesn't know he's a Thing making a noose, or he's a Thing knowingly making a noose for sympathy. It could be the Thing making a noose so that Mac would think he's still too crazy to let inside, but secretly wants Mac to leave him so he can finish his Saucer.