How long after the big bang would the conditions be just right for the first black hole to form ?
Given the density of energy shortly after the big bang I suspect some black holes would have formed almost immediately. I would think almost certainly some micro-sized ones did, but those would have evaporated by Hawking radiation.
Some cosmologists believe that some black holes may predate the Big Bang
http://www.technologyreview.com/view/423910/some-black-holes-may-pre-date-the-big-bang-say/
Others say supermassive black holes formed soon after it.