Here is the code from QDOS port modified to work with HoT. It adds the cvar v_contentblend. It functions just like gl_polyblend in that it disables any palette shifting or screen flashes.
Simply put this in your /engine/hexen2 directory and recompile clean.
The view.c from the link you gave is exactly the same version which is already in our 1.5.x releases, i.e. there's no v_contentblend thing in it.
Repaired the link, it's at the same URL, but in case someone is reading the bottom of the thread it's available at http://dk.toastednet.org/h2viewc.7z
The patch touches more places in hexen2/hw than its original quakeworld version.
Besides, the newly added cvar is not initialized properly according to the cvar api of uhexen2: the third member in the cvar_t structure is unsigned int flags, not a boolean, therefore you need CVAR_NONE instead of false in there (luckily, CVAR_NONE is 0 as is false, so your cvar behaved as you expected it to.)
At the moment, I am not interested in this change. (However, the information is here for those who are.)
Good catch on the CVAR_NONE, made a goof!
I updated the archive with the CVAR_NONE fix. In Q1 and QW it's just FALSE.