Iron can undergo fusion. However, iron is the point where fusions starts to cost more energy than it yields, so in a typical star it doesn't fuse.
In a supernova, and the abundance of energy available in one, iron will continue to fuse to heavier materials, which is probably how we got heavier metals here on earth in the first place (it has to have fused somewhere, after all.)
More to the point, in the case of black holes, when they collapse at first, see <PERSON> answer, because he explained that bit better than I could.
After it is a black hole, it becomes impossible to talk about it's composition in terms of mass and energy that we are used to. While we're not sure about electrons (AFAIK), Protons and Neutrons have a hard diameter. Removing the space between them causes for example a Neutron star, which you could consider one very, very large atom, from a certain point of view:
Most of the basic models for these objects imply that neutron stars are composed almost entirely of neutrons (subatomic particles with no net electrical charge and with slightly larger mass than protons); the electrons and protons present in normal matter combine to produce neutrons at the conditions in a neutron star. Neutron stars are supported against further collapse by neutron degeneracy pressure, a phenomenon described by the Pauli exclusion principle, just as white dwarfs are supported against collapse by electron degeneracy pressure.
Source: https://en.wikipedia.org/wiki/Neutron_star
However, at a certain pressure, even the neutrons/protons cannot uphold more matter that way. 70
That's when we get black holes. We know they're more compact than neutron stars, but we don't even know how big they are - only that they're smaller/equal to their event horizon. We don't even know if they HAVE a size.
So black holes are really 2 steps behind of where you could call a bunch of matter "Iron". Step 1 is the neutron star, where we already lost all information about what those particles USED to be before it was one. And step 2 is the black hole, where we don't even know what those particles/matter/energy currently are. Probably not anything we've seen so far.
Most of the essentials here can be found at wikipedia:
* https://en.wikipedia.org/wiki/Black_hole
* https://en.wikipedia.org/wiki/Degenerate_matter 781