See the description of Mind Blank in the statistics of the Elder Brain (VGM, pp. 173-174):
The older brain magically emanates psychic energy. Creatures of the choice of the older brain within 60 feet (…)
The only conditions specified for the destination are:
- It has to be a creature.
- The older brain has to pick it, which means it has to be aware of the presence of the creature.
- It must be within 60 feet of the older brain.
Rules for the spellcasting target in the PHB state the following:
To be able to target something, you must have a clear path to it, so that it is not behind the total cover. If you place an effect area at a point that you can not see, and there is an obstacle such as a wall between you and that point, the point of origin arises on the near side of that obstacle.
How Mind blast is not necessarily a spell that I can not say with certainty that this rule applies.
This would amount to the dm's decision.
If the DM allows it, it creates a very interesting combination with the older brain creature spirit Feature. Allows your enemies to be removed from security as long as it is completely shielded.