I remember reading (a very long time ago) that the best way of getting close to pure black was to take a horn (of the type used to store gunpowder back when fli

I remember reading (a very long time ago) that the best way of getting close to pure black was to take a horn (of the type used to store gunpowder back when flintlocks were considered modern weapons) and hold the open end over a smokey candle until the entire inside was covered in lamp black. The combination of the very non-reflective lamp black and the curved shape of the interior ensured that almost all the light entering didn't leave.