That's why "blame" is in quotes. He's not literally blaming himself. But since you presumably have to start out the communication with a statement of its purpose, he takes that opportunity afforded by the first sentence, to frame the whole message in terms of "help me understand" instead of "you suck."
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!