I think that was a bad call because I don't like how they interpretted the rule, BUT the exact same call (both on interceptions) went against the Steelers twice in the regular season under the exact same logic (against the Colts and against the Bengals, back-to-back weeks we had that call go against us, though the penalized guy was a receiver trying to make the post INT tackle instead of the QB, that's the only difference). So while I disagree with the rules interpretation that was consistent with how they've interpretted the rules all year. So I can't call it a "good" call but I will say it's not a "bad" call because it's consistent.
How can a (now) defensive player BLOCK illegally? Chop the knees out from under his own guy trying to stop the interception from scoring? I am not saying that poor officiating was the only reason the Steelers won, but it was not insignificant.