Chicago is entrusting $20 million to the last black-owned bank in the city, Illinois Service Federal Savings and Loan Association, thereby bolstering a lender that began in the Great Depression as officials join a nationwide movement to steer idle funds to underserved communities. “If we’re going to be serious about supporting those communities and community banks and what they do for small business, we have to look for opportunities like this,” said city treasurer Kurt Summers.