Line 453. Do you think Hooper will reveal why he has worn the black viel for so many years?

I think he reveals his reasons in the excerpt below. The black veil is a symbol of sin, all sin, not a specific sin. He wears publicly what the parishioners secretly hide. But none-the-less, in the very end, he tells them they each have a visage (face) with a black veil.

"Why do you tremble at me alone?" cried he, turning his veiled face round the circle of pale spectators. "Tremble also at each other! Have men avoided me, and women shown no pity, and children screamed and fled, only for my black veil? What, but the mystery which it obscurely typifies, has made this piece of crape so awful? When the friend shows his inmost heart to his friend; the lover to his best beloved; when man does not vainly shrink from the eye of his Creator, loathsomely treasuring up the secret of his sin; then deem me a monster, for the symbol beneath which I have lived, and die! I look around me, and, lo! on every visage a Black Veil!"