The head of the Maritime Administration. Robert J. Blackwell, said yesterday that he has turned down a $110,000-a-year job offer from a group of nine ocean shipping companies.

Blackwell, who has been assistant secretary of commerce for maritime affairs since 1972, came under fire last week when the job offer was disclosed at a House Merchant Marine and Fisheries Committee hearing under questioning by Rep. Paul N. McCloskey (R-Calif.).

The Commerce Department official denied there was anything more than "the specter of an impropriety" hanging over the offer that he head a prospective new trade association for the nine firms. He insisted that he had successfully insulated himself from any conflicts of interest while he mulled over the offer.

The firms included eight federally subsidized shipping Lines: American Export Lines. Moore-McCormack Lines, Prudential Lines, Farrell Lines, Delta Stcamship Lines, Lykes Bros. Steamship Co., American President Lines, and States Steamship Co. In a June 24 memo to Secretary of Commerce Juanita Kreps, Blackwell listed the companies and said he would "immediately refrain from doing business" with them and would "avoid all personal and business contacts with representatives of the companies."

He said yesterday that he felt free now to resume dealings with those companies as federal maritime administrator and would resume his chairmanship of the three-member maritime Subsidy Board. Blackwell said he had disqualified himself from the board, which makes "almost" all decisions affecting the subsidized lines.

The ninth shipping company that might have participated in the trade association, Blackwell said yesterday, was the non-subsidized United States Lines.

More McCormacks and United States Lines were accused in a federal indictment Monday of having bribed former Rep. Edward A. Garmatz (D-Md.), who served six years as chairman of the House Merchant Marine Committee, in exchange for legislation worth $24 million to the two firms.