Hundreds packed into a Jacksonville City Hall chamber Friday night to honor African-American “trailblazers” who had overcome racial barriers in fields from medicine and education to publishing and politics. “It is an incredible feeling to know that I am in the room with so many giants,” Mayor Alvin Brown told an estimated 450 people who gathered to celebrate 17 people who built careers in fields there once all-white. “You have set the course for our future,” said Brown, the city’s first African-American mayor.