Ah yes, the puddings, everyone's favorite oozes (only not at all). Would you believe that the CR 5 blood pudding resembles blood? What a shocker! Even more shocking is what it feeds on: blood! In seriousness, though, the blood pudding seems to be purely designed for gore factor. Their main method of attack is siphoning into another lifeform and then forcibly expelling said lifeform's blood out from any available orifice. In a supernatural horror campaign, especially one in a more modern setting, this could actually be used in interesting ways. What if, rather than manifestations of bigger supernatural phenomena, things such as stigmata or "tears of blood" are actually signs someone is being slowly eaten alive by a blood pudding?
Would you believe that the stone pudding has a touch that can turn people to stone? Shocking, I know.
Now here's something you old school D&Ders might be familiar with: quasi-elementals! Basically, to make a long story short, quasi-elemental planes are where Negative and Postive planes meet elemental planes. The quasi-elemental planes of classic Gygaxian canon are the Quasi-Elemental planes of Lightning (Air and Positive), Vacuum (Air and Negative), Mineral (Earth and Positive), Dust (Earth and Negative), Radiance (Fire and Positive), Ash (Fire and Negative), Steam (Water and Positive), and Salt (Water and Negative). In the first Tome of Horrors, only the lightning quasi-elemental was detailed. That means that we get vacuum, radiance, etc. elementals here in the ToH II, right?
Sadly, no. What we do get are the elementals of two new quasi-elemental planes: the Quasi-Elemental Plane of Acid and Obsidian. Acid quasi-elementals are basically very caustic water elementals, having the Acid rather than Water subtype. The Acid subtype is vulnerable to water damage, which dilutes and injures the acidic creature, and Acid subtype creatures tend to (surprise surprise) deal acid damage. In the case of acid quasi-elementals, they have both acid damage upon their drenching touch as well as a leaking of noxious fumes that deal Constitution damage.
Obsdian quasi-elementals, by contrast, are more or less alterations of earth elementals, even still having the Earth subtype rather than a new one. The main difference with the obsidians is that their partially fiery nature means that if they are wounded, they "bleed" molten glass in potentially damaging spurts, and explode into numerous spiky shards upon death.