Interesting link ...
Other variables such as ambient cooling due to airflow, tube orientation and correct biasing in power tubes have a greater influence on the plate temperature than the colour of the plate.
My sentiments exactly, which is why I went the direction I did on my custom chassis, adding clearance and making the bias easier to monitor and control. BTW, none of my "go to" tubes are black plate, although I have several in the collection.
... it’s possible that plate material might have some fundamental effect on the tone. Or perhaps something else in manufacture of black plate tubes affects inter-electrode capacitances, transconductance, plate resistance or leakage – all real measureable effects.
Seems likely that different materials and processes would have different results in what otherwise are electronically identical tubes. HOW a tube dissipates heat is a big factor as well, and glass thickness and formulation, as well as something as simple as the material used in the pins would logically affect performance and longevity.