It's true that a hollow conductor has a minimum cutoff frequency.
However, a hollow conductor is not a black body.
A black body has perfect absorption of radiation at all frequencies, while a perfect conductor perfectly *reflects* all radiation.

A black body emits radiation according to the [Planck law](https://en.wikipedia.org/wiki/Planck%27s_law).
Since the black body absorbs all incoming radiation, but then re-emits it according to the Planck law, it must be capable of converting from e.g. a single high energy photon to several lower energy ones.
The hollow perfect conductor can't do this because it reflects all radiation without ever absorbing and "processing" it.

For more details I strongly recommend reading the other Physics Stack Exchange post [Why is black the best emitter?](http://physics.stackexchange.com/questions/159856/why-is-black-the-best-emitter) and the associated answer.