I don't have access to the blackguard rules directly, but I gathered the basics from other people's posts. They get thier vice striker mechanic plus thier dark menace mechanic, but both only work on Paladin powers. HP, NADs, Profiencies, surges are like a normal Paladin hybrid. Hybrid talent feats are Blackguard Armour Prof and Shadow Shroud. You lose out on a Blackguard improved SS, Shadow Leap, Shadow infusion, temp hp from an ally spending a healing surge beside you. Most other Blackguard servant of vice and vice's reward are leveled utility powers so you can still take them. You can take a use of Dread Smite when ever you could get an encounter power. So you could stack dread smite on Howling Strike and even though Howling strike is a barb power, dread smite is a pally power so you could benifit from Menace and your vice mechanic, such as scarificing temps for extra damage.
I want to play a Blackguard/Ardent in the future, awesome augmentable atwills to blend with dread smite, a cool strikey leader ability with Mantle of Elation, plus you can go charisma primary and dump strength, because Ardents are a melee charisma weapon class. Your Blackguard/Barbarian would be cool too, but I'd go with fury for thematic reasons. Howling strike plus dread smite and blackguard striker features would be killer. HP would still be very high too. Blackguard/Warden with a focus on Divine Sanctions would be a decent striker, but a kick ass defender.
Oh you can I believe pick a pally non blackguard power as your at will. So say virtue strike for an Adent/BG, or Ardent strike so you can divine sanction with your at will for Barbarian or Warden. Blackguard/Warden really does rock as defender this way, although cavalier/Warden maybe better I don't know.