Currently, Transmute Blood will have zero effect on the Blood Homunculus, and if you try to cast it on one, you get the feedback text saying "this spell does not work on creatures that don't have blood." Which is silly because the Homunculus is literally made out of blood. Transmute Blood should be a reasonably devastating spell to cast on them. This would be in line with other spells that have a fun side-effect on stone golems.
In the same vein (har har), it'd be neat to have some secondary effect when cast on a hemomancer (maybe reducing blood stacks) or vampire (malus to blood meter). Blood effects are cool and fun, and I think there's a lot of potential there.