Here are my thoughts:
I think there are three ways you can approach blood. This is because blood magic is all segregated out into its own path. Using blood incurs a large research overhead! Therefore if you are not a strong blood nation you probably do not want to bother with blood research unless you really haven't anything else to do. Therefore, either you are a real blood nation, you are using blood only to forge, or you are not using blood at all. Which pretenders/nations/themes are best used in each role is left as an exercise for the reader
I like the Soul Contract, although its value is greatly reduced if you haven't got a Dwarven Hammer. A handful of devils are just strong flying units, but a horde of them can be devastating. If you are a real blood nation, you can conjure up a Horde from Hell and use the devil commander to lead the devils that the Soul Contract produces. Otherwise just use any undead leader. Personally I prefer to put the Soul Contract on a stealthy unit and send it along with the group. That way you don't have to worry about losing your income of devils if you happen to lose a battle somewhere along the way.
A horde of vampires MIGHT have a chance of killing a non-undead SC because of their undead/life drain combo. On the other hand, ethereal units have difficulty engaging SCs because they all have magic weapons. The SC will kill one vampire per attack unless by some miracle he misses. So you are looking at losing 4-6 vampires every round. You'd need a whole bunch and you are going to win by fatigue, not by outfighting the SC. If you are fighting in friendly dominion, of course, you don't have much risk with this approach.
Ghosts - mostly useful because of ethereal and fear - neither works against SCs.
Devils - maybe. Devils are expensive to throw away in this fashion, even if you have soul contracts. Devils are part of this complete breakfast.
Wights I prefer to use as "shock troops" since they have excellent (though not mindless) morale, good protection, and bane blades. Unless you can cast Legion of Wights, they are too expensive to produce in "horde" quantities. The trouble is they don't fly and are exceptionally slow so the SC will get to pick and choose who to attack. He will probably attack the commander. Although you might look at a bunch of normal wights as a way of increasing the effectiveness of a Bane Lord SC, by giving him extra distraction and firepower.