Captainkilner,
Anything that is vunerable to heat damage can only be protected from fire for so long. This is a problem in data storage safes that are "fireproof" and I guess this would apply to black boxes as well. See, the only defense from being in a fire is insulation, and no insulation is perfect. You can have a coolant that is used (some safes have a moist material in the layers that evaporates to move the heat away). But that means mass and mass is an important consideration.