The following works with/without enumitem, but only pertains to the first level of enumeration:

\documentclass{article}
%\usepackage{enumitem}
\makeatletter
\newcommand{\bolditem}{{%
\renewcommand\labelenumi{\bfseries\arabic{enumi}.}% Update label to bold
\item\leavevmode% Set item
\xdef\@currentlabel{\@currentlabel}% Preserve label (for cross-reference purposes)
}}
\makeatother
\begin{document}
Before the list
\begin{enumerate}
\item First item
\bolditem Second item
\item Third item
\bolditem Fourth item
\end{enumerate}
After the list
\end{document}
\bolditem temporarily redefines the way the enumeration label is printed, then sets a regular \item and reverts back to the default definition.