Formation of Black Holes in First Order Phase Transitions
  A new mechanism of black hole formation in a first order phase transition is proposed. In vacuum bubble collisions the interaction of bubble walls leads to the formation of nontrivial vacuum configuration. The consequent collapse of this vacuum configuration induces the black hole formation with high probability. Observational constraints on the spectrum of primordial black holes allow to obtain new nontrivial restrictions on parameters of inflation models with first order phase transitions. 
M.Yu. Khlopov, R.V. Konoplich, S.G. Rubin, and A.S. Sakharov