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most important question: what wavelength does your IR light have?
https://en.wikipedia.org/wiki/Infrared#Regions_within_the_infrared
apparently the ink from your printer absorbs visible light (you see it's black), but infrared only barely (nearly as bright as plain paper in your IR image).
you need something that absorbs your infrared illumination. "carbon black" is commonly used. take the soot from a lit candle to blacken the surface (a stencil may be involved). there are also inkjet inks that absorb infrared. I don't know if toner from a laser printer would work.