An effort encouraging investors to buy stock in Black-run banks could create new challenges for the leaders of those companies. The Buying Black movement, which took root last week, led to sharp increases in the shares of companies such as Broadway Financial in Los Angeles, Carver Bancorp in New York and M&F Bancorp in Durham, N.C. An influx of new investors could increase pressure to improve shareholder returns, while any strategic effort designed to generate higher profits could also draw a backlash. At the same time, markets are fickle — most shares in Black-run banks have fallen significantly in recent days as some existing shareholders cashed out.