Obsidian uses MathJax JavaScript library to render inline and displayed LaTeX equations embedded in your documents. MathJax supports most of the standard TeX and LaTeX math syntax, as well as some AMS and other LaTeX extensions and custom macros. See MathJax TeX and LaTeX Support page for details. Please note that MathJax doesn’t provide a full LaTeX environment. Only math-mode macros are supported.
In your case you can use \boldsymbol{x} or mathbf{x}. You also can define the \bm command somewhere in your document
$$\newcommand{\bm}[1]{\boldsymbol{#1}}$$
and use it later:
$$\bm{x}$$
Also see