Typically a color transmission densitometer will read in three color channels and a visual channel (B&W density). The blue channel read out is more accurate then the visual channel when one is using a staining developer such as Pyrocat since it indicate the additional density imparted by the proportional stain. Additionally other densitometers such as the Xrite 361T will read UV which is even more accurate when one is printing alt process with a film that has been developed in a staining developer like Pyrocat. I imagine that Phil Davis uses a black and white densitometer of some type (I have not seen the video that you mention). The reason that I think that is most likely is that Davis at latest report did not support staining developers in his research.