Bold square brackets

\usepackage{bm} and then \bm{[}. Maybe this isn't bold enough for you in which case you can use a font with a heavy variant (eg the commercial mathtime set) and \usepackage{mathtime,bm} and then \hm{[}.

Otherwise maybe you can use a different symbol, such as double square bracket \llbracket from \usepackage{stmaryrd}.