Black and white emphasis for \boxed?

You can use the predefined boxes from the fancybox package, which you can wrap around equations via the empheq package:

\documentclass[letterpaper]{article}
\usepackage{kpfonts,amssymb,empheq,fancybox}

\newsavebox{\mybox}

\newcommand{\raisedshadowbox}[1]{%
\sbox\mybox{\shadowbox{#1}}%
\raisebox{-0.5\ht\mybox-0.5\shadowsize+0.6ex}{\usebox\mybox}%
}

\begin{document}

A double box

\begin{empheq}[box=\doublebox]{align}
a&=b\\
E&=mc^2 + \int_a^a x\, dx
\end{empheq}

An oval box (cheesy)

\begin{empheq}[box=\ovalbox]{align}
a&=b\\
E&=mc^2 + \int_a^a x\, dx
\end{empheq}

A shadowbox, which has some vertical alignment problems \ldots

\begin{empheq}[box=\shadowbox]{align}
a&=b\\
E&=mc^2 + \int_a^a x\, dx
\end{empheq}

\ldots which apparently can be fixed by sticking it into a raisebox. 

\begin{empheq}[box=\raisedshadowbox]{align}
a&=b\\
E&=mc^2 + \int_a^a x\, dx
\end{empheq}

\end{document}

This gives

Note that the definition of \raisedshadowbox contains a fudge factor of 0.6ex for the vertical offset correction. I don't know what this compensates for, but it does seem to work across different font sizes, line heights as well as \shadowbox, \fboxsep and \fboxrule settings.

Some alternative examples made with tcboxmath, ams equation|gather|align|... tcolorbox environments:

\documentclass[letterpaper]{article}
\usepackage{lmodern}
\usepackage{amssymb,mathtools}
\usepackage[most]{tcolorbox}

\tcbset{colback=white,colframe=black}

\begin{document}

With \texttt{tcboxmath} only the equation is boxed
\begin{equation} 
\tcboxmath{\frac{M}{R} \propto R^2 \rho}  
\end{equation}

An \texttt{ams equation} also boxes equation number
\begin{tcolorbox}[ams equation]
 \label{eqn:density} \rho \propto \frac{M}{R^3}  
\end{tcolorbox}

With \texttt{tcbhighmath} some part can be detached
\begin{tcolorbox}[ams gather]
\frac{M}{R} \propto R^2 \rho \\
\tcbhighmath[colback=black,coltext=white,colframe=white]{\rho \propto \frac{M}{R^3}}
\end{tcolorbox}

Boxes can have shadows:

\tcboxmath[enhanced, drop shadow]{\frac{M}{R} \propto R^2 \rho}  \hfill
\tcboxmath[enhanced, drop fuzzy shadow]{\frac{M}{R} \propto R^2 \rho}\hfill
\tcboxmath[enhanced, drop lifted shadow]{\frac{M}{R} \propto R^2 \rho}  

sharp corners
\begin{tcolorbox}[ams equation, sharp corners]
 \label{eqn:density} \rho \propto \frac{M}{R^3}  
\end{tcolorbox}

border lines
\begin{tcolorbox}[enhanced, ams equation, sharp corners, borderline={1pt}{-3pt}{black}]
 \label{eqn:density} \rho \propto \frac{M}{R^3}  
\end{tcolorbox}
\dots
\end{document}