For me it has more to do with resale. If nothing ever escapes the collection, then it's a black hole even if some of the art is displayed or discussed publicly
Yep. Like Daren just said, the term was coined by pieces going in but never escaping. Once it enters the black hole it's gone forever from the market.
If you can see the art publicly, then it is not "inescapable."
I definitely disagree with this part. There are countless pieces of art on public display that can't be touched with any amount of money. Likewise, public offers are very effective at drawing out the unseen art.