Blackstone Group is selling its stake in a Santa Monica office complex for more than $500 million.

The New York-based mega investment firm is selling to Boston Properties Inc., a real estate investment trust, according to sources familiar with the transaction. The Colorado Center property spans six office buildings over 15 acres. Blackstone bought a 50 percent stake in the site in 2007 from Equity Office Properties Trust, part of a $39 billion buyout of the company.

Colorado Center’s tenants include cable TV giant Home Box Office Inc., video streaming service Hulu, ad firm RPA, and cyber security firm Symantec Corp.

Equity Office bought the property for more than $443 million in 2004, according to CoStar Group.

The sale comes as Blackstone continues to sell off property bought in the Equity Office deal. Last March, it sold four Westwood office towers to Douglas Emmett for $1.34 billion.

A Blackstone spokeswoman declined to comment on the transaction. Boston Properties did not respond to a request for comment.

The news was first reported by The Real Deal.