You’ve found an error in the bbm
manual. They meant \boldmath
, a standard command to select \mathversion{bold}
. So, you could set your title formatting to \bfseries\boldmath
or \fontseries{bx}\mathversion{bold}\selectfont
. This would give you bold math symbols to match your bold text.
A more-convenient way to select the bold version of a single math symbol is the \boldsymbol
command from amsmath
:
\documentclass{article}
\usepackage{amsmath, bm, bbm}
\newcommand\mathbb[1]{\mathbbm{#1}}
\newcommand\mathbbb[1]{\boldsymbol{\mathbb{#1}}}
\begin{document}
\begin{gather*}
\mathbb{N}\mathbbb{N} \\
\mathbb{R}\mathbbb{R} \\
\mathbb{C}\mathbbb{C} \\
\mathbb{Q}\mathbbb{Q}
\end{gather*}
\end{document}

This aliases the bbm
commands to more-standard ones used by other packages, such as mathalpha
.
You can switch to sans-serif by changing the definition of \mathbb
to
\newcommand\mathbb[1]{\mathbbmss{#1}}
Which gets you

Note that the bbm
fonts are in an obsolete format, and will be pixelated in a PDF file. If you want bold double-struck letters, you can load an OpenType math font with a bold version in unicode-math
(such as Libertinus Math or XITS Math), and \boldsymbol{\mathbb{N}}
will work. In PDFTeX, you can load
\usepackage[bb=pxtx]{mathalpha}
This supports \mathbbb{N}
if you select a \mathbb
font that comes in bold, and will get you a Type 1 outline font instead of a bitmap.