In the sin-obsessed atmosphere of the Puritan era, the Reverend Mr. Hooper's black veil, which hides the lower portion of his face and casts doubt in the minds of his congregation that their minister may be hiding some dark secret, casts a portentous gloom upon the wedding ceremony when he arrives.

This congregation of Puritans who carry an unforgiving view of human nature wonder at their earlier church service what their minister is hiding behind "his two folds of crape [crêpe]." The "gloomy shade" that Mr. Hooper continues to wear to a funeral and now to a wedding is very unnerving to the people.

Each member of the congregation, the most innocent girl, and the man of hardened breast, felt as if the minister had crept upon them, behind his awful veil, and discovered their horrible iniquity of deed or thought. 

Thus, when the minister arrives at the wedding wearing the dark veil over his face, the mood of the gathering is greatly affected. Whereas the guests have supposed that the minister would discard his veil and adopt his usual "placid cheerfulness" for this happy ceremony, he instead enters wearing this small pall-like cloth. Thus,

...a cloud seemed to have rolled duskily from beneath the black crape and dimmed the lights of the candles.

To the wedding guests it appears that the pale bride resembles the maiden who has been buried only a few hours before, and this deathlike maiden is one and the same.
So powerful an effect has this black veil of the minister produced that even when he catches a glimpse of himself in a looking glass, he himself shudders with horror, spills his untasted wine, and rushes out into the dark night. "For the Earth, too, had on her Black Veil."

The minister's black veil casts a pall upon the occasions Mr. Hooper attends so that the funeral differs little from the wedding. From his donning of this veil, then, there evolves a feeling of dread in the congregation whenever they encounter the Reverend Mr. Hooper, on whose face they can only detect the "glimmering of a melancholy smile," and he is avoided. 

The black veil that Mr Hooper dons has a massive impact on the wedding. Remember that this is the first day that he has worn the black veil, and the congregation think that after the funeral service, which of course was an appropriate service to wear a veil to, Mr Hooper will reconsider his attire for the wedding. However, he does not, and it is clear from the narrator's comments that the guests and the bride and groom themselves are affected greatly by the veil. Note the following description:

Such was its immediate effect on the guests that a cloud seemed to have rolled duskily from beneath the black crape, and dimmed the light of the candles.

Not only does the black veil therefore apparently darken the area around Mr Hooper in the church, even going as far as to "dimming" the candlelight, it also turns the bride into a very pale and trembling figure, so much so that some members of the congregation believe her to be the young woman whose funeral was celebrated just a few hours before. The black veil therefore is seen as a bad portent for such an occasion, and it clearly affects the ceremony.