Equation caption format: Strange parenthesis?

Here it is, with the \newtagform command from mathtools:

\documentclass[12pt,a4paper]{article}
\usepackage{blindtext}
\usepackage{mathtools}
\renewcommand{\theequation}{\Alph{equation}}
\newtagform{sans}{\sffamily\bfseries\footnotesize(})

\begin{document}

\usetagform{sans}
\blindtext
\begin{equation}
    a^2 + b^2 = c^2
\end{equation}

\end{document} 

I don't know if this is the most correct way of doing this, but...

The macro that holds the format of the equation tag is \maketag@@@. The standard definition is \hbox {\m@th \normalfont #1}, where #1 is (\theequation).

Adapting it to your example:

\documentclass[12pt,a4paper]{article}
\usepackage{blindtext}
\usepackage{amsmath}
\makeatletter
\renewcommand{\theequation}{\Alph{equation}}
\renewcommand{\maketag@@@}[1]{\hbox {\m@th \sffamily\footnotesize\bfseries #1}}
\makeatother

\begin{document}
\blindtext
\begin{equation}
    a^2 + b^2 = c^2
\end{equation}

\end{document}