  We show that primordial black holes can be produced from the collapse of
large isocurvature perturbations of the cold dark matter. We develop a novel
procedure to compute the resulting black hole abundance by studying matched
perturbations of matter-only universes, and we use our procedure to translate
observational constraints on black hole abundances into model-independent
constraints on cold dark matter isocurvature perturbations over a wide range of
scales. The constraint on the typical amplitude of the primordial perturbations
weakens slightly slower than linearly on small scales.
