Mind Blank as listed would for sure stop the Truesight spell, because it is a divination spell and per the phb pg 259- 260
one willing creature you touch is immune to [..], Divination Spells
but the Monster sense of Trusight is not a spell (Divination or otherwise). It is a special sense, per MM pg 8-9
[..] Special senses are described below.
and so on...
But the tricky part is that the Mind Blank spell also says
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.
So, are a monster's special senses "effects of similar power used to affect the target's mind or to gain information about the target"?
This becomes a big question when players start using Mind Blank in conjunction with Greater Invisibility.