After defeating Silver and viewing the sequence with Nemesis, the boss battle against Apocalypse is supposed to begin.

Instead I get a black screen; is there a known fix for this?

There is an other thread about cinematics not being played. I thought there was only the one at the telescope but Satellight mentioned some cinematics at the end. Maybe your problem is related to this. Did you try to sit it through or press some buttons that would normaly skip a cinematic?

This is not a cinematic; this is the boss fight itself.

The only button that has any notable effect when it's blackscreened is Esc, which returns me directly to the desktop.

I am also having this problem.

I beat silver, and the screen just goes black before the final battle begins.

I have no idea what the problem could be.

Old post, but I had the same problem with blackscreen and acces violation on top of that. Use a potion of absolute protection just before Silver is defeated. This should help you avoid the blackscreen and trigger the last fight.

Worked for me :)