Colors of statusbars items in irssi are defined in default.theme file in irssi configuration directory. It is a plain text file and you can edit it. You can also create your own theme file.
In particular, to change foreground color for statusbar from white to black, find this line:
sb_background = "%4%w";
and replace it with:
sb_background = "%4%k";
The full list of color codes is available here: https://github.com/irssi/irssi/blob/master/docs/formats.txt