The way to define current reference string for your own LaTeX environment

That is, the way to define output style of \ref{foo} when \label{foo} is within your own environment.

\newcounter{sidebar}[chapter]
\renewcommand{\thesidebar}{\thechapter.\arabic{sidebar}}
\newenvironment{sidebar}{%
 \refstepcounter{sidebar}}{%
 ...
...}

\refstepcounter is essential. It is this command that defines the current reference string which is available as \thesidebar command ('sidebar' is that in 'thesidebar'). All you have to do is renewcommand this \thesidebar command to your favorite style like above example.