@Mech, Given that the return loss is 0.2dB, the end of the transmission line is effectively short or open, the 180 degree phase shift indicates a short (open would start near 0), and the constant group delay across frequencies means its either a very very well built lumped sum delay line, (with a shorted output) or a length of transmission line. Walks like a duck, talks like a duck, till I hear otherwise, I suspect the duck is a witch.
If said black box is a piece of RF equipment, and indeed the test cable wasnt cal'd out, and an input amplifier went short, even if it was AC coupled with a large enough coupling cap, you'd get exactly what is being seen here. Or if said box was quite large, and had coax meandering though it to get to where it needed to go, there could easily be 80cm of coax.