The woman who appeared to be escorting Blunt away from the ceremony was dressed almost but not quite exactly like his wife.

This suggests that the arresting agents took note of what everyone wore to the rehearsal (that was officially but not actually canceled.) They then had a near duplicate outfit made to be worn by the woman arresting him. The subtle differences in the outfits would allow the team making the arrest to tell which woman Blunt was with at what time.