(Reuters) - Shares of BlackRock , the world's largest investment manager, have had a tepid go of it. They are up only 4 percent this year, versus the nearly 14 percent gained by the S&P 500. But the stock is poised to rally in the next year, possibly handing investors a 16 percent return, according to a story in Barron's on Sunday. Shares have underperformed, Barron's says, in part because of the way Vanguard and Charles Schwab charge lower fees. There is also the rampant speculation that BlackRock founder and Chief Executive Laurence Fink may become the next U.S. ...