TOKYO, April 27 (Reuters) - Japan's Nikkei share average slipped 0.4 percent on Friday after the Bank of Japan's move to boost its asset purchases by 10 trillion yen ($124 billion) failed to ease concerns among investors over the fragile economy. The index has lost 5.6 percent this month and marked its worst April performance in seven years, after rallying more than 19 percent in the first quarter. The benchmark Nikkei was down 40.94 points to 9,520.89 after jumping as high as 9,691.70 shortly after the BOJ announcement. The broader Topix index fell 0.7 percent to 804.27.