Black tie for a wedding is almost indescribably vulgar,regardless of the desperately aspirational children's names. If you must wear a suit, single breasted suit in any color except black, with a single vent is suitable, however morning dress ( which is entirely separate from mourning dress) is just about acceptable if still a little crass, if that is what the groom's party is wearing and you have asked permission.
Morning dress is traditionally a gentleman's riding attire for a morning in town, and is appropriate for wear until 4pm, or if a change of attire is not available, 7pm at the latest, at which point the gentleman should retire to his accomodation and return suitably equipped. Sitting for dinner or supper in morning attire is an absolute no no.
Hope that clarifies the situation for you.:)