Kind of like the glass-half-full or glass-half-empty quandary, a blank page can feel either optimistic or foreboding, depending on your perspective. In Wao, Díaz uses the blank page to symbolize the latter: the absence of what's supposed to be there. In this novel, it represents how totalitarian states control history.
When a dictator is in office, a lot of things get left out of the history books. The dictator strikes his awful offenses from the record, of course. And that's pretty much all there is to tell about these epochs, so those pages of history end up blank for a while. Here's a particularly good (or should we say bad?) example:
Considered our national "genius," Joaquín Balaguer was a N****phobe, an apologist to genocide, an election thief, and a killer of people who wrote better than himself, famously ordering the death of journalist Orlando Martínez. Later, when he wrote his memoirs, he claimed to have known who had done the foul deed (not him, of course) and left a blank page, a página en blanco, in the text to be filled in with the truth upon his death. (126.96.36.199)
This evil dude Joaquín Balaguer taunts the DR with the blank page. He puts it right under their noses just to let them know that he doesn't have to fill it in. Díaz's book itself, though, serves as the anti-blank page.
Since the history books of totalitarian regimes leave the victims' pages blank, it's a good thing our dude Díaz's around to fill them in. He tells the story of the Cabrals and of Oscar. (Even though Oscar is victim of the police after Trujillo, we all know that Trujillo's regime never really ended.)
Díaz writes all over those blank pages, granting space for Oscar and his family to tell their own stories. Furthermore, even though the capitán kills Oscar, Oscar wins in the end. Oscar experiences love; his last (written) words are "The beauty! The beauty!" (3.final letter.4). This seems like a victory to us.
So we think that Díaz isn't just retelling Trujillo's victims' stories; he's attempting to rewrite history. He's attempting to tip the odds in the favor of the victims, and in the favor of the forces of good.