How about this?
\colorbox{black}{%
\begin{minipage}[b]{\dimexpr\textwidth-2\fboxsep\relax}
\strut\color{white}hello
\end{minipage}%
}

A new environment to do the equivalent is
\setlength\fboxsep{0pt}
\makeatletter\newenvironment{blackbox}
{\noindent\begin{lrbox}{\@tempboxa}\begin{minipage}[b]{\dimexpr\textwidth-2\fboxsep\relax}\strut\color{white}}
{\end{minipage}\end{lrbox}\colorbox{black}{\usebox{\@tempboxa}}}
\makeatother
which will give the same result with
\begin{blackbox}
hello
\end{blackbox}
Note that this leads to an overfull box if unless \setlength\fboxsep{0pt}
is used before it. With that, the output looks like
