I added the "Blood" levels for three creatures in DefaultStats and specified what each level gives, just like for other Orcs.
I also added the "Blood Fury" ability...
But Wyverns don't gain "Blood" and, consequently, don't level up their "Blood" levels.
However, if a Wyvern has the "Blood" buff (I have a workaround that applies the 1st level of "Blood Fury" to creatures), it receives its deserved stat buffs for that "Blood" level.
So, the settings in DefaultStats work. But the "Blood Fury" ability doesn't seem to work for them.
Maybe someone knows something about this or has tried to do something similar. Perhaps there's a registry somewhere where they need to be registered for this feature. I searched using the TotalCommander search, but I couldn't find anything (there's one mention, but I checked it, and it's not what I'm looking for).