Primordial black hole formation caused by the QCD transition ?
  We consider the evolution of cosmological perturbations at the QCD transition, in particular the sudden reheating from a supercooled universe to the transition temperature. Sudden reheating happens at a specific temperature, hence density, and singles out one specific hypersurface. Underdensities reach the reheating earlier than overdensities, there is a short period of huge pressure differences which leads to a jump in the fluid velocity. Density perturbations of scales far below the Hubble radius $\lambda\ll R_H $ get an amplification which grows quadratically in wavenumber. Primordial black hole formation will not be sufficiently amplified by the QCD transition unless the initial spectrum is fine tuned. 
Peter Widerin, Christoph Schmid