When a black hole swallows a portion of matter, that matter ceases to have any energy. Its energy falls to absolute zero. This means that for that portion of matter, entropy falls down to zero.

What about the 2nd law of thermodynamics? It states that entropy of the universe can only increase with time. Do black holes contradict that law?

It turns out they do not! Black holes cause matter-antimatter particle pairs to occasionally split up into single particles. In such a pair, the particle which escapes the event horizon contributes to what we call "Hawking radiation".

Hawking radiation is perfectly random - that means maximum emtropy. That is what makes the 2nd law of thermodynamics hold just as well for black holes that locally decrease entropy to zero.

Let us imagine a clock falling down into a black hole. What would we observe on the clock in the moment it crossed the event horizon? It would slow down to the point of stopping.

Assume we could still observe the clock after it crossed the event horizon. What would we see on the clock? We would see that it started to move backward in time!

From our point of view, time runs backwards inside a black hole! This means that matter inside a black hole is time-reversed. What is time-reversed matter? We call it anti-matter.

Each positron swallowed by a black hole moves backward in time. We may view it as its anti-particle moving forward in time, and then the positron travelling backward through time becomes an electron travelling forward through time from the black hole.

The entropy of that electron is contributed back to the universe and that way the 2nd law of thermodynamics is obeyed.

References

1. Clean Coder Video Series episode 46: "Birth of Agile", 2018 ^[1]