In se’ifim 9-10, AH cites the custom to recite the blessing with a cup of wine or beer in hand, as we do to solemnize all sorts of ceremonies (like a berit milah). When we have such a cup, we make the blessing on the cup first, then the kiddushin. If no wine or beer is available, it is not essential to the ceremony, and then we would just recite the blessing itself.
Two unusual issues with this cup: first, we generally expect a person who recites a birchat ha-nehenin, a blessing made before enjoying a physical pleasure, to indulge that pleasure itself. Here, although the person conducting the ceremony recites the blessing on the wine, custom has only the bride and groom drink from it. AH thinks this is fine, because that’s how it was instituted, although he concedes it would be better if the mesader kiddushin, the rabbi performing the ceremony, drank. [When I was in yeshiva, we were told some rabbis would be careful to spill a bit of the wine on their hands as they passed the cup to the bride and groom, and then slurp that. AH thinks it unnecessary.]
Second, there is no need here to drink the majority of the cup, contrary to most situations of kos shel berachah, the cup accompanying a blessing. This is a cup to be tasted, not drunk.
