Why is the standard enthalpy of formation of black phosphorus not zero?
Reproducibility is more important than stability. To obtain black phosphorus, you have to heat your sample under high pressure for quite a while, and even then it may still contain a significant number of random crystal defects, so its properties are not quite the same each time you make it. White phosphorus, on the other hand, is prepared by sublimation and hence considerably pure.

That's why white phosphorus, rather than black, is assumed standard and has zero enthalpy of formation.