Wow, that is tin foil hat territory. [Yes, I know the foil isn't tin anymore.] However, the Faraday cage test is probaly OK.
I have some near IR CCDs (response to 1um) and R72 filters. Looking at black stuff, you find black often isn't black under IR. Some black is completely transparent to IR, so I have to presume there are different ways to produce black items. Maybe the black that isn't transparent to IR has carbon in it.
According to this wiki (gasp), carbon black absorbs IR: