This Portal for Mac update includes a fix for the black portals. Steam auto-patches games, so all you need to do is restart the Steam client and wait and the fixes will be downloaded and applied.
This is BrainLog, a blog by Dan Sanderson. Older entries, from October 1999 through September 2010, are preserved for posterity, but are no longer maintained. See the front page and newer entries.
May 15, 2010