Seems to me that we are talking about 2 different kind of "blackhole collectors" here:
Type 1: People that never had an online gallery (for any reasons) or that don't have time to update their galleries, or that are lazy to do it. (add here more similar reasons)
Type 2: People that used to have an online gallery but one day they decide (because of different reasons than what you could consider on Type 1) to remove it, hide it or whatever you call it.
I personally only considered to be "blackhole collectors" the second kind, i mean, those who pourpousely remove their galleries and don't want their cels to be displayed for everybody to watch anymore. But well, seeing the above comments i agree that people may want apply the term to many more kind of collectors out there.
I am a "blackhole collector" myself, from type 2 no less. I wrote about this several times on this board in the past so i don't feel like going again into details of the reason i chose to be this way. Lets just say that now my life as cel collector is "easier" than before. Of course that means sacrificing certain things like interacting with other collectors but oh well.....
But, that doesn't mean i don't have a gallery. I do have one on rubberslug but the cels on public display are very few (basically the ones i'm, willing to part with if anyone makes offers), but the rest is for my eyes only. Like other people here i always enjoy taking a look at my own gallery 24/7, seeing the cels all together

, as having to go physically browsing cel books is not really very practical. Again, like others have mentioned, the gallery also has insurance pourpouses.
With that said, i have almost always given the password of the gallery to whoever asked for it. I don't mind people taking a quick look at my collection. I just change the password after a few days. At the beggining i had to give the password to quite a lot of people, but after a few years of hiding the gallery, the number of people that remembered what my gallery looked like decreased to the point that now it has been several years without having to change the password
