A lot of things have been said about black hole from the early unbelief to the too strong belief. Here we will recall some of the principal feature of black holes before studying the constraint.
A well known fact about black hole it is their tendency to attract all things that come too near, even light therefore their name of black hole.
From very general argument one can find some constraint on black hole such, like it is proved in [20], the minimal mass is 
, the maximun charge is |Z|=30... This come from the fact that the Compton wavelenght of particles must be smaller than the Schwarschild radius of the black hole for the minimal mass or because the electrostatic force for Z higher than 30 will stop the collapsing .
The problem with the study of black hole is that there is no direct observation. We can know the existence of a black hole by its effect.
When talking about black hole we generally separate them in two class.
Like one can see this distinction do not have a very sharp criteria.
In gravitationnal collapsing there is formation of a singularity but behind an horizon (we do not have a naked singularity).
One fact about primordial black hole is that they can go through inflation and stay trapped! The inflation period does not change the fact that we have an hidden singularity, a black hole will not be opened by inflation .