BROCKTON — To Ruthie Harris of Brockton, Barack Obama's message of change is more important than his historic nomination as the nation's first presidential candidate of color. "Of course, we know it's history," said Harris, a Brockton business owner, community activist and African-American. "It's not the black thing with me, I just need to see the economy turn around." After a week-long display of unity that spotlighted not only Obama and his vice presidential choice, Delaware Senator Joseph Biden, but New York Senator Hillary Clinton and former president Bill Clinton, Democrats look ahead to the campaign against Arizona Senator John McCain, the presumptive Republ