There are plenty of opportunities to do this when giving criticism/feedback. Not only does it make it easier for the other person to hear, but it also helps you save face if your feedback ends up being wrong.
Rather than "blame", I suppose we're to say Allen accepts responsibility for his confusion and makes an effort to enrol others in his attempts to seek clarity. That way the opportunity arises for any involved to say "ah, I see, thank you".
If we hear what people are saying with a sense of goodwill, and read their words charitably, we give less cause for animosity to arise.
FWIW I don't see this as placing blame on himself, at all: I see it as Allen just stating a fact. Allen is confused and seeking clarification. Confusion can happen for all sorts of reasons with varying parties or circumstances being the cause.
Allen might similarly say "I'm confused" upon reading the time cube website, but Allen's not to blame for that.
I agree with you though that largely politeness comes from being internally focused ("I don't understand, I don't agree, I didn't know") as opposed to externally focused ("You did something confusing, You did something wrong, You didn't tell me"). One instance where it pays to be selfish, I suppose!