The black hole was discovered in 1976, about 30 years ago. Obviously the black hole is much older than that as we're seeing the light as it was 30 million years ago, but the light we're seeing is from when the black hole itself was only 30 or so years old.
My understanding is that scientists have seen black holes forming previously, but none so close. I was listening to the radio and not paying a lot of attention, but I thought they said they usually pop up at several billion ly away, which means that we usually see them in forming in other galaxies.
The supernova was observed 30 years ago. That it led to a black hole was confirmed recently. Not all supernova make blackholes.
As for the ones you refer to, we've not "seen" them - it is believed that certain types of gamma ray emissions are due to the birth of black holes. We've detected these emissions, but never been able to associate a particular emission with a visual observation of a supernova or other black-hole forming event. This is the first time (that I've heard of) where we have a visual event where a black hole could be found, and for which we have the gamma ray evidence to support black hole formation.
Bryan
