Nothing needs to be that dark, but the possibility does exist for something to be that dark. With how strange black holes are I really wouldn’t be surprised if something that makes up a black hole also happened to be able to absorb light. We can only see the event horizon as radiation is able to escape it. The rest is just unknown about the core past observations.
The intensity of the gravity is able to pull everything in. The intensity of the spin gives the event horizon the disc like shape, but also it would appear that it’s also magnetic too.
It would appear that black holes aren’t just 1 thing and to put them in only 1 box might be bad for our understanding of them.