\blacksquare in .sty file

Question

I am planning to have a .sty file for scibing notes. When modifying one that already exists online, I found that I could not use

 \renewcommand{\qedsymbol}{\blacksquare}

or

 \renewcommand{\qedsymbol}{\ensuremath{\blacksquare}}

either to change the usual white square to the black one. So, how would I do this?

On a similar note, if I wanted to add a blacktriangle, how would I do this?

In case, the answer depends on how the rest of the .sty file looks, I am most willing to point to the file I am modifying.

My .sty file:

 \newtheoremstyle{myplain}      {10pt}{10pt}{\itshape}{}{\scshape}{.}{.5em}{}
 \newtheoremstyle{mydefinition} {10pt}{10pt}{}{}{\scshape}{.}{.5em}{}
 \newtheoremstyle{myremark}     {10pt}{10pt}{}{}{\scshape}{.}{.5em}{}

 \renewcommand{\qedsymbol}{\blacksquare}   %replacing this line with 
                                           %\renewcommand{\qedsymbol}{\ensuremath{\blacksquare}} 
                                           % does not work either.

 \newcounter{lecture}
 \theoremstyle{myplain}
 \newtheorem{thm}{Theorem}[lecture]
 \newtheorem{lem}         [thm]{Lemma}
 \newtheorem{pro}         [thm]{Proposition}
 \newtheorem{cor}         [thm]{Corollary}
 \newtheorem{claim}       [thm]{Claim}
 \newtheorem{fact}        [thm]{Fact}

 \theoremstyle{mydefinition}
 \newtheorem{defn}       [thm]{Definition}
 \newtheorem{eg}         [thm]{Example}
 \newtheorem{assumption} [thm]{Assumption}
 \newtheorem{openproblem}[thm]{Open Problem}
 \newtheorem{problem}    [thm]{Problem}

 \theoremstyle{myremark}
 \newtheorem{remark}        [thm]{Remark}
 \newtheorem{conjecture}    [thm]{Conjecture}

 \numberwithin{equation} {lecture}
 \numberwithin{figure}   {lecture}
 \numberwithin{table}    {lecture}

 \renewcommand{\fnum@figure}{\textsc{Figure~\thefigure}}
 \renewcommand{\fnum@table}{\textsc{Table~\thetable}}

 \renewcommand{\thesection}{\thelecture.\arabic{section}}
 \renewcommand{\thepage}{\thelecture-\arabic{page}}

Answer 1

You probably forgot to load amssymb:

\documentclass{amsart} % just to have `amsthm` preloaded
\usepackage{amssymb}
\renewcommand{\qedsymbol}{$\blacksquare$}
\begin{document}
\begin{proof}
a
\end{proof}
\end{document}