Looks like I'm not the only person thinking about Johnny Carson, which isn't surprising, and some of the thinking has led people to find fascinating footage. Andrew Hearst's excellent new site, Panopticist, hosts video of Johnny Carson doing two cringeworthy and revealing pieces--one a sketch where he appears in blackface, and the other (and more disturbing, if you can believe that) is Carson interviewing a black actor and becoming increasingly jaw-dropping...it's rather chilling, what we accepted, and in many ways still accept. Download the video here--it's worth the bandwidth.
Hearst had the footage because it was originally compiled by Jamie Greenberg for his now-defunct media skewering show, Media Shower--Hearst wrote a piece on Media Shower in 2000, which is available online. Daniel Radosh encouraged Hearst to put the film up, and his commentary is here.