An important feature about black hole was proved by Hawking [22]. Black hole emit radiation and therefore are not really black. They radiate high energy particles; they evaporate.
The evaporation time (in seconds) is given by 
where M is in g (like always for that kind of data).
To evaporate now, a primordial black hole
must have a mass of 
. There is no particular reason for a black hole to evaporate now but if one does, then we will be able to observe it. This will give a lot of data on black holes and also for high energy particle physics.
As a result of that evaporation we can have a lot of observable effect of black hole.
Then constraint on black hole mass come from very various observations such as the microwave background , entropy production ... This lead to a very complete set of constraints that you can see in the table 
Table: Constraint on the mass spectrum of primordial black hole.
A good way to visualize how all the result from the different sets of data can be put together is shown in the figure (for more detail about it see [29]).
Figure: Upper limit on 
the fraction of the universe that form primordial black hole
of that mass from different kind of measurement. M is normalized to gramme
In that figure there are data from the measurement of the total density parameter 
, the 
-ray background, the primordial helium He and Deuterium D abundances and the photon-to-baryon ratio S.