From the Mind Blank spell's description:
The spell even foils wish spells and spells or effects of similar power used to affect the target's mind or to gain information about the target.
Hence, a creature under the effects of Mind Blank should intuitively be immune to Madness from p. 259 of the DMG.
As answered by "Does the Mind Blank spell prevent the target from being frightened?", however, unless an effect explicitly targets the mind (as Feeblemind does) or other points mentioned in the spell's description, it seems like Mind Blank would offer no protection against the effect (e.g the aforementioned question answers that Mind Blank does not protect against being frightened). Certain types of madness lie within this category, such as:
31-40: The character becomes frightened and must use his or her action and movement each round to flee from the source of the fear.
From p. 39 of the Curse of Strahd adventure, however:
The Mad Mage has forgotten his name and the world whence he came. In fact, he doesn't remember anything that happened before the madness… Under normal circumstances, a greater restoration spell cast on the Mad Mage would restore his wits and end the madness… But in this case, the Mad Mage has cast a mind blank spell on himself. As long as that spell remains in effect, his sanity can't be restored by any spell.
There are two interpretations of the above example:
- A creature's current sanity/madness level is an aspect of the mind protected by Mind Blank. Therefore although certain effects of madness may not explicitly target the mind, so long as it is a type of madness it is protected against by Mind Blank.
- Mind Blank does not protect against all types of madness, but the above example happens to be one of which that is protected since the text explicitly details its interaction with Mind Blank.
As the above is the only example I know of where madness and Mind Blank interact, I have very little data to go on.
Are there any other examples from published sources or rules I missed to help explain whether Mind Blank protects against all types of madness?
Best Answer
Mind Blank may not make you immune to madness, but it blocks all effects that cause Madness.
1. CoS Example
As your example mentions in CoS, Mind Blank prevents all spells that affect the target's current state of mind. The key point is separating the idea of immunity to madness from immunity to effects that cause you to go mad. The reason the character remained insane was because he did not have Mind Blank active at the moment he became mad. He only casted Mind Blank after he went mad because the madness made him:
If Mind Blank made you immune to madness, he should have been cured as soon as he casted Mind Blank. Remember, however, Mind Blank's description does not mention that it cures madness, it merely states that the spell prevents anything from further affecting their mind. In essence, the spell preserves the target's current state of mind. If Mind Blank did not protect the target's current state of mind, then the text would not explicitly state that:
Therefore although Mind Blank does not make you immune to madness, any effect that would cause it or cure it is prevented by Mind Blank.
2. RAI: If it Quacks Like a Duck...
This idea that madness is a state of mind is further reinforced by the definition of "Madness" according to Merriam Webster:
Many effects that induce madness not only require wisdom/intelligence saving throws, it also deals psychic damage. For example in IDRotF page 174:
As mentioned in Does the Mind Blank spell protect against the secondary effects of the Tasha's Mind Whip spell?:
Therefore madness is not only colloquially defined as a mental state, effects that cause the state to present itself deals psychic damage and requires mental saving throws. As the old adage goes, "If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck." These all imply that you must affect the mind to inflict madness.
3. Madness as Described in DMG page 259
The strongest argument for this interpretation, however, is probably the Madness descriptions in the DMG, where inflicting madness is stated to be imposing something upon the mind explicitly (i.e affect the mind).
Later descriptions also mention keywords that supports this interpretation:
As DnD also uses natural language (so that sourcebooks and adventures do not read like your programming textbook), the mentioning of "breaking your psyche" also reinforces the interpretation that inflicting madness is a processes affecting the mind.
In conclusion
Though Mind Blank does not make you immune to madness, if no effects can cause you to go mad, it basically serves as effective immunity (so long as you are not already mad before casting the spell). It essentially protects your current state of mind. You remain sane if you were already so when you cast the spell, and likewise if you were already insane as seen in the CoS example.