I don't know if I'd even categorize Black Forest as a "Blood Nation".

They start with a blood supercombatant and can go throughout a game without even researching blood if they don't wish while having a great use of their blood slaves.

They are by far the quickest blood nation and overall I personally feel strongest. They can get so may provinces so quickly they can create a massive blood foundation probably 5-10 turns before opponents. They don't need strong blood mages, as they can make blood items (which there are quite a few) with such a strong blood economy. Their real 'limitation' is the forced death scale, which makes you pillage provinces for blood slaves and have no way to compensate for it. However, by the time you are doing this, you have so many highly equipped Vampires, Devils, running around, with a very, very strong army in the rangers (with Wind Guide and Flaming Xbows, look for indeps or use fortune tellers) and powerful communions making them able to use many different spells. Their only real downfall is being able to search for whatever elements they need, which you can do by building the right kind of pretender with the magics you need.

Black Forest is easily in my Top 3 picks of the game in it's current state. If you are their nearest enemy, you had better hope you get paralyze before they get a count, and you better get storm or arrow fend soon after.