Basically if you want to invest in the stock market but are somewhat unsure about how to best approach it, you get recommended a passive fund. This fund only buys the stocks that composes a certain index (say SP500) and so has basically the same returns and risks. This in turn attracts much more people that wouldn't otherwise touch the stock market, and Blackrock ends up owning a huge portion of the stocks on the market instead of individuals. And with Blackrock's ownership of said stock comes voting rights in the respective companies.

And presto, they now control those companies.