As the housing market cools, more borrowers are facing foreclosure, especially in neighborhoods where high-cost loans abound. That spells trouble for Boston’s black and Latino homeowners. While only a small percentage of petitions lead to foreclosures, economists view the increase as a bellwether of financial distress. In a hot housing market, borrowers can sell their homes to escape financial trouble. But when housing prices stop going up, says Jim Campen, an economist at the University of Massachusetts at Boston, “they can’t.”