BlackRock has used its heft to promote policy change in the past year, including voting for a shareholder resolution that required the gun maker Sturm, Ruger & Company to be more transparent about the safety of its products. In some respects, Mr. Fink is limited in what he can do. Much of BlackRock's holdings are through 401(k) plans in index funds, and the company isn't able to sell specific companies whose policies it might disagree with. But it recently introduced a series of socially responsible investment funds that exclude entire industries, such as tobacco, firearms or coal. The letter is also a defense against those who criticized him over last year's letter. "I didn't know Larry Fink had been made God," the real estate billionaire Sam Zell said the day after Mr. Fink's letter was sent last year. And Warren Buffett, the chairman of Berkshire Hathaway, said he did not believe it was the role of investors to push their views in the way Mr. Fink suggested.