Investment management behemoth BlackRock ( BLK) is benefiting more than most from the equity rally that stocks have been enjoying for the past couple of quarters -- and the stock shows it. Shares of the $41 billion firm have rallied more than 38% in the last six months alone. BlackRock is the biggest money manager in the world, with approximately $3.8 trillion in client assets in its book. Because the firm earns its keep based on that AUM number, ballooning equity values expand BlackRock's revenues in kind. While BlackRock started off as a fixed income shop, it transformed itself after the Great Recession created an opportunity to buy Barclays Global Investors at a discount. Now, equities make up almost half of the firm's total assets under management, a fortuitous change in allocation given the performance stocks are seeing right now. Stocks also come with heftier management fees, and bigger profits for BlackRock's income statement. More recently, BlackRock's investment menu has expanded to include all sorts of alternative investments and ETFs, products that make BLK's grasp on AUM a bit more tenuous (low-risk strategies tend to have stickier assets) but are necessary for the firm's strategy. Despite ramping up the number of retail investors it serves, BlackRock still gets two-thirds of its assets from fellow institutional investors. While institutional clients are stickier, they also come with smaller fees and fewer growth opportunities. Retail customers are BlackRock's biggest growth opportunity for the next few years.