Rambam, Laws of Marriage 3:23, nonetheless applies the rules of birchat ha-mitzvah to this blessing, requires it to be said before the kiddushin [I once forgot to; read on for what options an incompetent rabbi such as myself has in that situation. I also just recently was at a wedding where the rabbi made the blessing on the wine after this blessing, I think also an error]. In his birchat ha-mitzvah framework, a blessing not made before is lost, would be a wasted blessing if made after.
Ra’avad and others think the blessing is only properly made after kiddushin, because we make blessings before a mitzvah only where we are the only human actors, so we have more control of whether the mitzvah comes to fruition, Hagahot Maimoniyot points out. Here, the woman might change her mind and reject the ring, wasting the blessing.
In a responsum, Rosh accepted Rambam’s view, which would imply that we see this as a birchat ha-mitzvah despite all we have said until now. Except that we do not follow Rambam’s rejection of saying the berachah after if it was forgotten before. As long as the couple is still under the chuppah, says AH, the berachah can be made, because it’s a blessing on the sanctity of the Jewish people.
