Blackboard bold

From ConTeXt wiki

Blackboard bold is probably the most common way of denoting the "standard" number sets: the natural numbers, the integers and so on. In AMS-LaTeX, you use the \mathbb{...} command to obtain them; the ConTeXt way is the \blackboard{...} command. Also, the following commands are defined:

\naturalnumbers
\integers
\rationals
\reals
\complexes,
\primes

\blackboard{F}
\blackboard{O}

See also Doublestroke for an alternative to the msbm font.