It is - quite literally - all relative. There is, from one point of view, a certain logic in having a boost gauge with what might be considered an arbitrary zero point. Most of us live at or relatively near sea level, so using ISA as zero is a commonsense approach. And speaking colloquially, 'boost' implies getting something extra, so a positive boost pressure, meaning a manifold pressure greater than ISA, is quite easy to understand. Where it falls apart though is when we talk of 'negative boost'. Again, speaking colloquially, most people would probably interpret this as a 'negative pressure', and of course in
absolute terms there is no such thing. Negative values indicated by a boost pressure gauge just mean that the manifold pressure is below ISA, as the supercharger or turbocharger fights a losing battle against diminishing ambient pressure at altitude. There will still be some positive pressure in the manifold come what may. Pressure, in absolute terms, can never be less than zero, in the same way that temperature, on the absolute scale (Kelvin) can never be below zero. Zero Kelvin is when all molecular motion ceases and you can't get below that value. Zero (absolute) pressure is a total vacuum and you can't get lower than that either.
The great thing the rules of physics is that they apply whether you believe them or not!
