The Federal Reserve is considering an auction for a large portfolio of subprime-mortgage bonds and is consulting with BlackRock Inc. about the process, according to people familiar with the matter. The prospect of an auction comes after American International Group Inc. earlier this month formally offered $15.7 billion to buy the bonds from the Fed, which took over the portfolio in 2008 to help the government-controlled insurer stem a cash bleed. A sale would mark another step in federal regulators’ efforts to wind down bailouts and emergency-assistance programs set up during the financial crisis.