I'm no expert, but this seems to work:

\setupcolors [state=start, system=rgb]

\def\bw#1{\framed[background=color,backgroundcolor=black]{\color[white]{#1}}}

\setuppagenumbering[command=\bw,location={header,margin}]
\starttext
\input tufte
\stoptext


On Mon, Aug 24, 2009 at 12:32 AM, Vyatcheslav Yatskovsky <
yatskovsky@gmail.com> wrote:

> Hello,
>
> How to make page number as white digit in a black square? (And place it on
> the right of the page)
>

Since right is ambiguous(it could be header or footer) I think you will be
able to figure it out
http://texshow.contextgarden.net/cmd/en/setuppagenumbering

HTH