I'm asking how you expect an auditor to confirm the absence of something in a series of black boxes that a determined and skilled adversary would like to hide.

It's actually quite simple. You fail the audit and block the purchase at the first black box you find :)

You realize that's completely impractical and will result in the government buying nothing, correct?

So you complain about useless audits but also complain about actual audits.

Ok.

Go complain somewhere else because this discussion is not productive.

It's not productive because the two options you've presented do not exist in reality.