Mind blank has no effect on zone of truth.
Zone of Truth (PHB, p. 289):
Until the spell ends, a creature
that enters the spell’s area for the first time on a turn
or starts its turn there must make a Charisma saving throw. On a failed save, a creature can’t speak a deliberate lie while in the radius. You know whether each creature succeeds or fails on its saving throw.
Mind Blank (PHB, p. 260):
Until the spell ends, one willing creature you touch is immune to psychic damage, any effect that would sense its emotions or read its thoughts, divination spells, and the charmed condition.
Zone of Truth isn't a divination spell (it's an enchantment) and doesn't allow the caster to sense emotions or read thoughts. Instead it magically restricts what an affected creature can say; rather than detecting lies, it just prohibits them from being spoken. The effect that lets the caster of zone of truth know whether a target has saved or not also doesn't meet any of these criteria. So, mind blank confers no benefits of any kind when being subjected to a zone of truth; a mind blanked character still needs to make a saving throw, and is subject to the effects of the spell if they fail.
Would the mind blanked creature be aware of the zone of truth?
An affected creature is aware of the spell and can thus avoid answering questions to which it would normally respond with a lie. Such a creature can be evasive in
its answers as long as it remains within the boundaries of the truth.
(Zone of Truth, PHB p. 289)
Any creature affected by zone of truth is aware of it; again, mind blank doesn't change this.