Poking around those articles (and knowing nothing really), it is interesting to note a couple references to a 50B solar-mass limit for “luminous accreting black holes hosted by disc galaxies.” (In your Phoenix cluster link). I guess these ones are easier to spot, based entirely on the word “luminous.”

There are other larger ones out there, looming in the darkness.

Those supermassive black holes are very old, from a time when the universe was much denser - they likely collapsed directly without any star formation