The coming transformation of ETFs into mutual funds.

At first glance, it seems like an unlikely marriage. Mutual fund leader BlackRock announced last week that it was purchasing Barclays Global Investors, which holds 49 percent of the exchange-traded fund market, for $13.5 billion. These have long been the opposite poles of investing: Most mutual funds try to make money by picking stocks, while ETFs try to make money by simply mimicking the market.

Perhaps the new megagroup will preserve both strategies. But it seems just as likely that BlackRock wants in on the business's quiet but growing trend called the actively managed ETF. If that sounds like a contradiction in terms, well, it is.

In simplest terms, ETFs are index funds—passive, diversified portfolios that trade like a stock. For the past decade, ETF providers like BGI have touted their products as the antidote to the overpriced, underperforming actively managed mutual fund. Over the past six years, investors invested fewer assets in mutual funds and more into ETFs. The trend accelerated during the financial crisis, as investors grew disgusted at the inability of their active mutual funds to protect their assets. Last year, equity mutual funds saw net cash outflows of $245 billion, according to TrimTabs Investment Research, while equity ETFs posted net cash inflows of $140 billion, even as asset values tanked. With all the negative feeling around actively managed mutual funds, why would the ETF industry step backward to make a big push for the actively managed ETFs?

For the money.

Index funds charge lower fees compared with active funds, which means less money in the manager's pocket. ETFs charge even less than comparable index mutual funds and offer the additional benefits of greater tax efficiency and transparency because they're structured differently. In addition, ETFs offer the ability to buy or sell shares during market hours. These reasons led ETFs to capture more than $500 million in assets and grab a significant market share from the $9 trillion mutual fund industry.

The first active ETF appeared early last year in an inauspicious start. Bear Stearns launched the ETF just weeks before the bank went belly up. The fund closed soon afterward. A short time later, Invesco PowerShares launched a family of five active ETFs. But they have found it difficult to gain wide acceptance and attract assets. The financial crisis effectively took these funds off most investors' radar.