Blackstone, the world's biggest private equity firm, is expanding to invest in companies that are too big for venture capital, but not mature enough for traditional buyouts. Jon Korngold, who spent 18 years investing in growth equity at General Atlantic, was tapped to lead the effort. Blackstone's other recent expansion areas include infrastructure, life sciences, and insurance, as the firm looks to more than double its assets under management to $1 trillion over time. Blackstone has hired a veteran who specializes in investing in fast growing companies to lead a new business.