A black hole swallows a star.
The star remains intact.
Then goes supernova.
What would happen to the star?
Could we detect this event?
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We could not-- whatever happens inside a black hole stays inside a black hole. One way this is often explained is that the time coordinate inside the "event horizon" of a black hole looks to us on the outside like a radial coordinate-- events that occur at sequentially different times inside the event horizon, such as your supernova, seem to us on the outside to happen at sequentially smaller radii, until the last event for your dying star is to reach the center. The same holds for all the ejecta and even the light and neutrinos emitted-- it all goes to the center in a finite time. So nothing escapes from the event horizon.
Another way to say this is that "black holes have no hair", meaning there is not information outside the event horizon stemming from anything happening on the inside. All that happens to the outside stems from what falls into the hole-- how much mass, charge, and angular momentum crosses the event horizon. What it does while it is in there has no effect on the outside, so could never be detected.