This error occurs because ‘BlackMesa.7z’ is either corrupt or incomplete. To verify this, simply download MD5 Checker, and compare the MD5 checksums with those listed in the ‘BlackMesa.md5’ file.
To do this, simply place ‘BlackMesa-Setup.exe’, ‘BlackMesa.7z’, and ‘BlackMesa.md5’ (all three files are available via the various mirrors in the download section of this very site) into the same folder. Open ‘BlackMesa.md5’ in Notepad and run MD5 Checker. Add ‘blackmesa-setup.exe’ and ‘BlackMesa.7z’. The app will automatcially generate the MD5 checksum for each file as they’re added.
Now right-click the entry for ‘BlackMesa-Setup.exe’ and select ‘Compare Checksum’ copy & paste it’s checksum in the ‘BlackMesa.md5’ file into the box. If they’re a match you should see ‘Same’ appear in the box next to the MD5 generated by MD5 Checker. Repeat this for ‘BlackMesa.7z’.
If a checksum doesn’t match, then that file is either corrupt or incomplete. You’ll have to re-download the file again. For reference, ‘BlackMesa.7z’ should be 3,298,098,513 bytes in size, and ‘BlackMesa-Setup.exe’ should be 827,720 bytes in size.