A paladin charger's barding requires the blessing of a powerful equine spirit. In the ruins of Dire Maul the corrupted treant known as Tendris Warpwood has such a spirit imprisoned. Seek retribution on behalf of this spirit, and free it from its bondage. With luck, it may be willing to provide its blessing. To help ensure success, I'd also advise you to bring a special type of spiritually-attuned feed. Travel to Eastern Plaguelands and speak with the ranger known as Nathanos Blightcaller about this.