BlueMountain Capital Management, a multistrategy and activist hedge fund firm founded by McKinsey & Co. alum Stephen Siderow and former J.P. Morgan trader Andrew Feldstein, has had only one down year since its 2003 launch. As a result, the firm, which specializes in equity and credit market investing, reappears on Alpha's Hedge Fund 100 ranking for the fifth time in seven years, sealing its highest position yet at No. 36. The New York–based firm first appeared in 2009 as the 92nd largest hedge fund in the world with $4.6 billion in capital. Since then the fast-growing firm has more than tripled its capital, raising $4 billion between 2013 and 2014 alone. Growth flattened in 2014, however…