What causes this black screen according to the guide?

The guide does not diagnose the cause. It gives a network workaround: two programs that change how the game's traffic is routed, which in the author's case lets the game reach the main menu after the intro scenes.

Which programs are needed?

Zapret (a Flowseal build for Discord and YouTube) and the Cloudflare WARP client.

Fixes & Performance

KF2 Black Screen at Launch: Zapret and Cloudflare WARP Fix

This page paraphrases a short Russian-language Steam guide on fixing the black screen that appears when launching Killing Floor 2. The author's workaround uses two network tools; it is meant for players whose connection to the game's services is being blocked or throttled.

Guide cover Cover of the original guide.

Programs

You need Zapret, from the zapret-discord-youtube project by Flowseal (the author says "I think this is the right one"), and the Cloudflare WARP client.

Setup

Zapret. After downloading, open service.bat and set the Game Filter option to TCP and UDP. Close the file, then run any working .bat file from the package.

Cloudflare WARP. Simply install it and switch on traffic and DNS handling.

Result

Launch the game and, after the intro scenes, you should reach the main menu.

Notes

This is a network-level workaround rather than a game fix. Third-party tools like these change how your whole connection behaves, so only download them from their official sources.
If your black screen is caused by something else, try the -nostartupmovies launch option or the other fix with the same tools in black screen with music: Zapret and WARP.

FAQ

What causes this black screen according to the guide?: The guide does not diagnose the cause. It gives a network workaround: two programs that change how the game's traffic is routed, which in the author's case lets the game reach the main menu after the intro scenes.
Which programs are needed?: Zapret (a Flowseal build for Discord and YouTube) and the Cloudflare WARP client.