Bolder Blackboard bold

Question

I am looking for bold versions of the blackboard bold letters. Unfortunately, the best symbols I have come up with so far are hard to distinguish from the usual blackboard bold letters. Is there a way to obtain even bolder blackboard letters?

My best attempt so far:

\documentclass{article}

\pagenumbering{gobble} % switch off page numbering

\usepackage{amsmath}
\usepackage[bb=boondox]{mathalfa} % for mathbbb

\begin{document}
\begin{align*}
    \mathbbb{B} & \mathbb{B} \\
    \mathbbb{C} & \mathbb{C} \\
    \mathbbb{N} & \mathbb{N} \\
    \mathbbb{P} & \mathbb{P} \\
    \mathbbb{Z} & \mathbb{Z}
\end{align*}
\end{document}

Answer 1

Based on the comments to the original question, I have compiled a list of options below. No option is perfect so far:

• "Bold" is not blackboard bold
• "mathbbb" and "kern" are both hard to distinguish from normal "mathbb"
• "kern" and "more kern" have a rendering artifact at the tips of "C"
• "contour" looks weird when scaled down a lot (but it is the best so far)

\documentclass{article}

\pagenumbering{gobble} % switch off page numbering
\usepackage[landscape,margin=1in]{geometry}

\usepackage{amsmath}
\usepackage[bb=boondox]{mathalfa} % for mathbbb
\usepackage{stix2}
\usepackage{xcolor}
\usepackage{contour}
\contourlength{0.02em}

\usepackage{amsfonts}

\newcommand{\mathbbkern}[1]{%
    \ooalign{$\mathbb{#1}$\cr$\mkern0.5mu\mathbb{#1}$}%
}
\newcommand{\mathbbkernmore}[1]{%
    \ooalign{$\mathbb{#1}$\cr$\mkern0.5mu\mathbb{#1}$\cr$\mkern1mu\mathbb{#1}$}%
}
\newcommand{\mathbbbkern}[1]{%
    \ooalign{$\mathbbb{#1}$\cr$\mkern0.5mu\mathbbb{#1}$}%
}
\newcommand{\mathbbbkernmore}[1]{%
    \ooalign{%
        $\mathbbb{#1}$\cr%
        $\mkern0.05mu\mathbbb{#1}$\cr%
        $\mkern0.10mu\mathbbb{#1}$\cr%
        $\mkern0.15mu\mathbbb{#1}$\cr%
        $\mkern0.20mu\mathbbb{#1}$\cr%
        $\mkern0.25mu\mathbbb{#1}$\cr%
        $\mkern0.30mu\mathbbb{#1}$\cr%
        $\mkern0.35mu\mathbbb{#1}$\cr%
        $\mkern0.40mu\mathbbb{#1}$\cr%
        $\mkern0.45mu\mathbbb{#1}$\cr%
        $\mkern0.50mu\mathbbb{#1}$\cr%
        $\mkern0.55mu\mathbbb{#1}$\cr%
        $\mkern0.60mu\mathbbb{#1}$%
    }%
}
\newcommand{\ccontour}[1]{%
    \colorlet{contour-saved}{.}%
    \contour{contour-saved}{#1}%
}
\newcommand{\mathbbcontour}[1]{
    \ccontour{\ensuremath{\mathbb{#1}}}
}
\newcommand{\mathbbbcontour}[1]{
    \ccontour{\ensuremath{\mathbbb{#1}}}
}

\begin{document}

\begin{align*}
    \begin{array}{ccccccccccc}
        \text{Normal} & \text{Bold} & \text{mathbb} & \text{mathbbb} & \text{kern} & \text{more kern} & \text{mathbbb + kern} & \text{mathbbb + more kern} & \text{contour} & \text{mathbbb + contour} \\
        %
        B & \mathbf{B} & \mathbb{B} & \mathbbb{B} & \mathbbkern{B} & \mathbbkernmore{B} & \mathbbbkern{B} & \mathbbbkernmore{B} & \mathbbcontour{B} & \mathbbbcontour{B} \\
        %
        C & \mathbf{C} & \mathbb{C} & \mathbbb{C} & \mathbbkern{C} & \mathbbkernmore{C} & \mathbbbkern{C} & \mathbbbkernmore{C} & \mathbbcontour{C} & \mathbbbcontour{C} \\
        %
        N & \mathbf{N} & \mathbb{N} & \mathbbb{N} & \mathbbkern{N} & \mathbbkernmore{N} & \mathbbbkern{N} & \mathbbbkernmore{N} & \mathbbcontour{N} & \mathbbbcontour{N} \\
        %
        P & \mathbf{P} & \mathbb{P} & \mathbbb{P} & \mathbbkern{P} & \mathbbkernmore{P} & \mathbbbkern{P} & \mathbbbkernmore{P} & \mathbbcontour{P} & \mathbbbcontour{P} \\
        %
        Z & \mathbf{Z} & \mathbb{Z} & \mathbbb{Z} & \mathbbkern{Z} & \mathbbkernmore{Z} & \mathbbbkern{Z} & \mathbbbkernmore{Z} & \mathbbcontour{Z} & \mathbbbcontour{Z}
    \end{array}
\end{align*}

\newpage

\begin{align*}
    \arraycolsep=2pt
    \begin{array}{cccccccccc}
        C & \mathbf{C} & \mathbb{C} & \mathbbb{C} & \mathbbkern{C} & \mathbbkernmore{C} & \mathbbbkern{C} & \mathbbbkernmore{C} & \mathbbcontour{C} & \mathbbbcontour{C}
    \end{array}
\end{align*}

\end{document}

Here is a magnified version of kern and contour versions of ℂ, to show the rendering artifact:

Answer 2

I’m not sure whether you’re asking for two different weights of blackboard bold in the same document, or for a heavier blackboard-bold font.

Selecting a Heavier Blackboard Bold

I would first check the font samples in the mathalpha manual, if you are using PDFTeX. If you can use unicode-math, you can select a different weight of \mathbb with a command such as

\setmathfont{KPMath-Regular}
\setmathfont{KPMath-Semibold}[range=bb]

If your math font does not come in a bold version, you can fake one with something like

\setmathfont{NewCMMath-Book}
\setmathfont{NewCMMath-Book}[range=bb,
                             FakeBold=0.05]

You can make the blackboard bold heavier by increasing the number after FakeBold=.

Using Two Weights of Blackboard Bold

The mathalpha package supports \mathbbb for bolder-blackboard-bold. It only works for certain fonts.

Many other packages, including unicode-math and bm, allow you to write \boldsymbol{\mathbb{N}}. This requires you to have a bold math version defined. Normally, your font package will do this for you. If the math font you load with unicode-math has the weights -Regular and -Bold, the package will automatically load the latter as your bold math font.

You can also set this up manually, with commands such as

\setmathfont{KPMath-Light}
\setmathfont{KPMath-Semibold}[version=bold]

In legacy 8-bit TeX, you would set the bold math version for blackboard bold with a command such as

\SetMathAlphabet{\mathbb}{bold}{xyz}{U}{m}{b}

In either case, you could for convenience define a command

\providecommand\mathbbb[1]{\boldsymbol{\mathbb{#1}}}