BlackRock will buy BGI for $13.5 billion in a mega-merger sure to send shock waves throughout the industry.

In perhaps one of the worst-kept secrets in exchange-traded funds industry history, giant asset manager BlackRock Inc. said late Thursday it had finalized a $13.5 billion deal to buy Barclays Global Investors.

The combined company, to be called BlackRock Global Investors, will represent nearly $3 trillion in assets under management.

Barclays will keep about a 20% stake in the new BGI.

BlackRock will only have to fork over about half of the estimated deal amount in cash; according to reports, BlackRock is exchanging shares of its common stock to complete roughly the other half of the mega-merger.

Officially, Barclays' original buyer (CVC Capital Partners ) has about a week to try to match BlackRock's offer. CVC Capital originally inked a deal to purchase most of BGI's ETF unit, iShares, in early April. The price tag at the time was listed at $4.4 billion, but it came with plenty of strings: Parent company Barclays was providing much of the financing, and it was retaining the rights to the share-lending business tied to iShares. The deal also had an escape clause for the seller if a better bid could be found by June 18. (See related story here.)

And that's just what happened. Nearly two weeks ago, reports started circulating that BlackRock had put together a package that would trump CVC Capital's offer, and would include not just iShares but the entirety of BGI, one of the largest institutional index managers in the world. The stories even detailed Barclays maintaining a 20% stake in the new BGI and many other financing terms. (See related story here.)

Then, last week, reports emerged that pretty much stated Barclays had settled on BlackRock. (See related story here.)

What will be the combined impact on the growing ETF market? Analysts immediately afterwards are looking at the world's largest insitutional money management firm and beefed-up ETF provider as dominant in both marketplaces. Perhaps the combined hulk of the new company will lead to further economies-of-scale in its management practices.

Others aren't so sure that such a merger will pay huge dividends, at least right away. Some industry veterans point to a shaky history for mega-mergers between asset managers and diversified financial services firms. (Going against that trend, of course, is the fact that BlackRock by most accounts has successfully integrated the former Merrill Lynch money management arm.)

Then again, a revamped BGI (along with its iShares ETF unit) faces a number of internal changes cutting at the core of its business. Consolidation has been taking place in both funds as well as asset managers, and many industry veterans are seeing fewer differences between the types of customers providers service in coming years. (See related column here.)

Still to be resolved as well -- a $175 million buyout clause that CVC is reportedly owed for breaking its original agreement.

In any event, the deal isn't expected to close, barring any unforeseen regulatory hurdles such a merger might raise, until late this year.