Gary Black, the chief executive of Denver-based Janus Capital Group Inc., resigned yesterday after what The Wall Street Journal calls a “roller-coaster” performance by the fund, noting some controversy surrounding his leadership. Black, who became CEO in 2006, issued a statement saying Janus is “in a much stronger place than when I joined,” adding that the time was right for change. Janus spokeswoman Shelley Peterson calls Black’s departure a “mutual decision between Gary and the board.” In the last year, Janus has suffered. Its Contrarian fund, for instance, was down 48 percent, and Black’s efforts to tie compensation more closely to performance was controversial with some. Bloomberg News reports that Black battled “to rebuild the mutual-fund company and cut the pay and influence of individual investment managers.” Tim Armour, a Janus director since March 2008, is Black’s interim replacement, according to the Denver Business Journal. Under Black, operating margins shrunk from 34.5 percent to 23.5 percent, writes The Denver Post.