A Boston-based real estate firm which last month was denied permission to build a 18-story hotel and 15 story office complex near Crystal City has filed a $6.5 million damage suit against the Arlington County Board.

The suit, filed in Arlington Circuit Court by North American Mortgage Investors, charges that the board's rejection of the 4.6-acre complex to be located at 2765 Jefferson Davis Highway amounts to "legislative blackmail."

Last month the board, which has been criticized by federal officials for approving high-rise developments that supposedly deface the Washington skyline, narrowly defeated the developer's proposal and later refused to reconsider that action.

The Boston firm originally had proposed that the board grant it permission for three stories above the county's height limit in exchange for street improvements and the construction of a pedestrian bridge linking the complex with the Crystal City Metro station.

In court papers, Carson Lee Fifer, the developer's attorney, noted that Arlington officials recently approved a proposal by Washington developer Kingdon Gould to build a similar and slightly taller office and hotel complex in the same area.

"As a result of the unconstitutional and unlawful action," court papers said, "NAMI has been denied the right to develop property (to) the best and highest use" in accordance with state law.

After last month's vote, board Chairman Dorothy T. Grotos who joined board members Ellen M. Bozman and John W. Purdy in opposing the project, said she felt that the real estate firm had not offered sufficient public improvements in exchange for additional height.