Beamer: Text Justification in blocks globally

you could load the ragged2e package and add a \justifying at the end of the block begin template:

\documentclass{beamer}
\usepackage{ragged2e}   %new code
\usetheme{default}

\addtobeamertemplate{block begin}{}{\justifying}  %new code

\begin{document}
    \begin{frame}{Test}
        \begin{block}{Test}
            The brown fox jumped over the lazy dog. The brown fox jumped over the lazy dog. The brown fox jumped over the lazy dog. The brown fox jumped over the lazy dog. The brown fox jumped over the lazy dog. The brown fox jumped over the lazy dog. The brown fox jumped over the lazy dog. The brown fox jumped over the lazy dog. The brown fox jumped over the lazy dog. The brown fox jumped over the lazy dog. The brown fox jumped over the lazy dog.
        \end{block}
    \end{frame}
\end{document}