\documentclass{article}
\usepackage{amsthm}
\usepackage{lipsum}


\newtheorem{proposition}{Proposition}

\newtheoremstyle{propositionExtrastyle}% 〈name〉
{0pt}% 〈Space above〉1
{0pt}% 〈Space below 〉1
{}% 〈Body font〉
{}% 〈Indent amount〉2
{\bfseries}% 〈Theorem head font〉
{:}% 〈Punctuation after theorem head 〉
{.5em}% 〈Space after theorem head 〉3
{}% 〈Theorem head spec (can be left empty, meaning ‘normal’ )〉

\theoremstyle{propositionExtrastyle}
\newtheorem{propositionExtra}{Proposition}

\begin{document}
\lipsum[1]
\begin{proposition}
Blah Bla Blah
\end{proposition}
\lipsum[1]
\begin{propositionExtra}
Blah Bla Blah
\end{propositionExtra}
\lipsum[1]
\end{document}