Blackening Materials

ELECTRODAG 501, Black electrically conductive coating has been used in space applications (Vacuum down to 10(-5) Torr.)for optical instruments on satellites for some years and may be of use for your application. The paint is qualified by ESA (European Space Agency) Noordwijk NL. to specification PSS-01-731. Further information : Solar absorptance is 0.965 the normal emittance is 0.829 and the surface resistivity is 1000 ohms for a thickness of 1 Mil outgassing is within ESA required limits.

Chemists use platinum-black on a platinum electrode for its large specific surface. It's black because the cristals are smaller then the specific depth of penetration for platinum at that wavelength and I know the effect from copper as well but there it is not stable. There is an electro-chemical procedure to make platinum-black. There is a reference to this in "reference electrodes" by Janz and Ives New York Accademic Press. It's too old for an ISBN. It contains two pages of further references.

You'd first have to make a platinum layer (on glass?). It depends very much on the surface material how to do that. You probably first make a copper or nickel layer and then a platinum layer. These are hazardous procedures and it would be wise to consult an electochomical lab You might try nickel black or chrome black too. Same idea, but you could go to a good harley plater and get it done much more easily that platinum.