TOKYO (Reuters) - BlackRock Inc has built a 5.21% stake in Toshiba Corp, according to a regulatory filing by the U.S. asset manager, after the Japanese industrial conglomerate returned to the Tokyo Stock Exchange’s top section. It becomes Toshiba’s third-largest shareholder behind hedge funds Effissimo Capital Management and Farallon Capital Management, a source familiar with the matter said. Toshiba, embroiled in a long-running row with activist shareholders including Singapore-based Effissimo, has said the return to the TSE’s first section would lure buying from investors in stock indexes. Toshiba rejoined Japan’s Topix index of top tier shares on Feb 25. The Japanese company was relegated to the second section in 2017 after massive writedowns at its U.S. nuclear power business caused liabilities to exceed assets - a condition for automatic demotion.