Even when they're presented with the naked truth, they would rather believe the lie. Or at least that it was too complicated to be figured out by a pair of boys after it confounded everyone for sixty years. Blind pride can be a useful thing.
We can parse "that it was too complicated ..." as the lesser alternative to "they would rather believe". The first assertion is that "they" would rather believe the lie. This is moderated to an assertion that "they" would rather believe that "it" was too complicated for the boys to figure out (than, presumably, that "they" were wrong).
If we adopt this parsing, the context indicates that "blind pride" was a label applied to the two boys:
The implication is that the boys were ascribed a baseless confidence in their own ability ("blind pride"). That is, the boys' disregard for others having tried for many years and failed was a "useful thing" because, after all, the boys succeeded. The boys wouldn't have succeeded if they had let history dissuade them from the attempt.
(Alternatively, "they" could be parsed to refer to the boys. It would be harder to account for "the lie"---perhaps referring to the boys' exaggerated opinion of their own ability---but the cartoon link suggests that the quote was uttered by the two 'boys', making "they" someone other than themselves.)