Score one for the machines.The largest fund company in the world, BlackRock, has faced a thorny challenge since it acquired the exchange-traded-fund business from Barclays in 2009.These low cost, computer-driven funds have exploded in growth, leaving in the dust the stock pickers who had spurred an earlier expansion for the firm. The rise of passive investing — exchange-traded funds, index funds and the like — has revolutionized the investment world, providing Main Street investors with greater opportunities at lower fees while putting pressure on even Wall Street’s biggest money managers.Now, after years of deliberations, Laurence D. Fink, a founder and chief executive of BlackRock, has cast his lot with the machines.