什麼是blackened thermometer ?

i guess you are doing physics questions on heat radiation.
it is for experimental use.
Objects of dark colours are good absorber and good radiator of heat.

I think you refer to a blackbody thermometer (黑球溫度計), which is an ordinary mercury-in-glass thermometer with the mercury bulb enclosed inside a blackened sphere.

A blackbody themometer measures the radiant temperature (輻射溫度) of the surroundings. The radiant temperature is one of four major factors (the other three are air temperature, relative humidity and air speed) that affect human comfort.