I know that Ignite Poison has some problems, but it is still one of my favourite spells. IMO a spell which is only powerful against a relatively small but still significant enemy set (poisonous creatures) is an interesting/unique effect. The convert poison/mephitic clouds into fire clouds effect is also nice, although the flame clouds disappear very quickly.
I would suggest keeping Ignite Poison, removing its rarely useful item-igniting effects and buffing cloud ignition instead (let the fire clouds have the same duration as the poison/mephitic clouds). This way the spell would be somewhat more useful against not naturally poisonous creatures. If it is still too weak against not poisonous creatures, buff its damage on poisoned monsters (and/or reduce the damage on naturally poisonous ones).
My problems with the suggested Ignite Blood are the following:
- Cold-blooded doesn't mean that the blood of the creature actually has low temperature, it simply means that creature doesn't maintain a fixed body temperature, so it shouldn't protect from igniting blood.
- Only affecting creatures with blood/living creatures is the territory of Necromancy/Poison and maybe Ice. Also, this is a serious restriction on a level 6 spell, especially in a school (Fire) where all other spells work on demons/undead/other weird creatures.
- I think two whole-los attack spells with self-damage (Toxic radiance, Refrigeration) are enough, we don't need one of them for every elemental school. If Ignite Poison is too similiar to Refrigeration, don't make it even more similar.
Mass sticky flame would be an interesting effect (invisibility reveal etc), I wouldn't replace Ignite Poison with it, but it could be a good level 6/7 Fire or Fire/Conj (or Fire/Hex?) spell (without the only living creatures limitation).