You mentioned that you are using the versions of Warcraft III + The Frozen Throne before Reforged, but please explain the actual version are you using in (and maybe where did you get it).
Black screen during the cinematics (before the game is fully loaded or the campaign itself) shouldn't stop you from doing anything. It's quite related to the modern operating systems, which doesn't recognize the video codecs made by Blizzard (it's made with DivX, if I remember correctly).
To fix the black screen during the cinematics, you'll have to do a little tricky thing. First, make sure you have LAVFilters, which you can get it from this post:
LAV Filters - Open-Source DirectShow Media Splitter and Decoders - Nevcairiel/LAVFilters
github.com
I highly suggest getting the installer version, as the manual method will only install the specific filters based on the OS-type you are currently using, which can led to a serious issues with your computer.
After that, run Command Prompt (must be with the Administrator, or else it won't work) by hitting Windows key + R and type in:
regsvr32 /u "C:\Program Files (x86)\Warcraft III\blizzard.ax"
This is assuming you installed Warcraft 3 in the default directory. If you installed it somewhere else, you'll need to edit the path to the correct directory. If you did it right, you will get a message that blizzard.ax has been successfully unregistered.
And that's it! If you followed every step, the cinematics should now play correctly within Warcraft 3. Bear in mind that the game will register blizzard.ax the first time you run it, so you must run the game at least once and then unregister the codec as written above, else you'll unregister it and the game will register it once you run it.
If you don't want a little tricky thing with LAVFilters, I really suggest getting K-Lite Codec Pack. This includes both LAVFilters and FFDShow, which solves the issues with your cinematics on Warcraft III:
Download links for the K-Lite Codec Pack. A free software bundle for high quality audio and video playback.
codecguide.com
Also, it is possible that you can actually watch Warcraft III cinematics without playing it. Just go to "Movies" folder and run any MPQ files using MPC-HC. The subtitles is already included by default.
