Ian Parker wrote:
Black holes for example, while having themselves lower entropy than environment,
are the largest catalysts of entropy increase because ~40% of matter
falling into them is converted into light.


I may be a little bit pedantic but do you mean light or gravitational
waves. If a mass falls into a static BH 50% of the mass is converted
into gravitational waves and 50% of it increases the mass of the BH.

A spinning BH is extremely interesting. Here up to 80% of rest mass
energy may go into gas jets. This is the main energy source of abnormal
galaxies.

I am refering to UV radiation caused by acceleration of charged
particles. Because the matter falling in the black hole stuff is
accelerated to near speed of light, this acceleration creates radiation
(of cause, that is before matter reaches the horizont of no return).
That is the reason why black holes are actually very bright in UV
and that is how they are commonly found.
This is just the basic idea, maybe specialists here can provide a
more detailed description of black hole UV brightness mechanism.

Regards,
Evgenij