The partial merger of Detroit’s two daily newspapers, the Free Press and the News, which was to have taken effect Monday, was blocked by Justice William J. Brennan Jr. until at least March 17, when the full Supreme Court meets at its regularly scheduled closed-door conference. Federal appeals court panels have upheld then-Atty. Gen. Edwin Meese III’s decision to allow the papers to form a joint operating agreement. Gannett Co., owner of the News, and Knight-Ridder, owner of the Free Press, say the papers are losing money. Opponents of the merger, calling themselves Michigan Citizens for an Independent Press, say the newspapers were unprofitable solely because both cut prices in an unsuccessful effort to force each other out of business.