  Primordial Black Holes (PBHs) might have formed in the early Universe as a
consequence of the collapse of density fluctuations with an amplitude above a
critical value $\delta_{c}$: the formation threshold. Although for a
radiation-dominated Universe $\delta_{c}$ remains constant, if the Universe
experiences some dust-like phases (e.g. phase transitions) $\delta_{c}$ might
decrease, improving the chances of PBH formation. We studied the evolution of
$\delta_{c}$ during the QCD phase transition epoch within three different
models: Bag Model (BM), Lattice Fit Model (LFM), and Crossover Model (CM). We
found that the reduction on the background value of $\delta_{c}$ can be as high
as $77\%$ (BM), which might imply a $\sim10^{-10}$ probability of PBHs forming
at the QCD epoch.
