Take that galaxy that just crossed our "observable universe horizon" so that we can't see it. If there is a civilization halfway between Earth and that galaxy, they can still see that galaxy. The galaxy can see that second civilization and so can we. There isn't a single fixed "observable universe" boundary in space, it's just relative to the observer.
With a black hole, it is different. There is no point that can see both sides while being seen from both sides,. If you are outside the black hole, you see nothing from within. If you are inside, then you can see the inside (this is speculation) and you also see the outside. It's a very clear boundary.
This is known to be not correct. When you are near the event horizon, you can still observe most of our ordinary observable universe, as well as subset of black hole's interior.
This is the direct reason/consequence of not being able to observe the event horizon when near it, or notice when you cross it.
The boundary is not clear, it depends on the observer.
Take that galaxy that just crossed our "observable universe horizon" so that we can't see it. If there is a civilization halfway between Earth and that galaxy, they can still see that galaxy. The galaxy can see that second civilization and so can we. There isn't a single fixed "observable universe" boundary in space, it's just relative to the observer.
With a black hole, it is different. There is no point that can see both sides while being seen from both sides,. If you are outside the black hole, you see nothing from within. If you are inside, then you can see the inside (this is speculation) and you also see the outside. It's a very clear boundary.