Let's see...try changing your output from openglnb to opengl or ddraw (just to discard that).
If you're using the latest DosBox (and you should) that .conf file has an invalid machine type.
As I said:

1#  machine: The type of machine tries to emulate.
2#           Possible values: hercules, cga, tandy, pcjr, ega, vgaonly, svga_s3, svga_et3000, svga_et4000, svga_paradise, vesa_nolfb, vesa_oldvbe.

So...change machine to

1machine=svga_s3

It works for X-Com.

I set machine=svga_s3
I set output=openglnb
I set scaler=none
The same blank screen appeared after choosing option 2 with these configuration options. Option 3 gave the same result.

Set output to ddraw?

Set output to ddraw?

I tried setting output to ddraw with the other settings remaining the same, but got the same blank screen.

Which .conf file are you editing and which DosBox are you running?
The status window says anything?

I'm using the .conf in H:\Games\Steam\SteamApps\common\XCom UFO Defense

The DOSBox status window reads: SHELL:Redirect output to nul //the above message repeats several times SHELL:Redirect output […]
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The DOSBox status window reads:
SHELL:Redirect output to nul
//the above message repeats several times
SHELL:Redirect output to scrap.txt
//more output to nul messages
SHELL:Redirect output to xcomutil.log
//repeats twice

I mentioned this on the first page of this thread. Also, there are messages that show up for a split second, but aren't written to the status window after the blank screen, and don't show up in screenshots regardless of how fast my reflexes are. "Unable" is the only word I can make out.

Meanwhile, at the Steam forums, a user mentioned that I should be launching it from runxcom. The file RunXcom.bat is located in three different places in my XCom UFO Defense folder. Here are the errors I get upon running each file:
H:\Games\Steam\SteamApps\common\XCom UFO Defense\XcomUtil\batch: "The system cannot find the path specified. Unable to find bit Files. Press any key to continue..."
H:\Games\Steam\SteamApps\common\XCom UFO Defense\XCOM\XcomUtil\batch: Same error as above.
H:\Games\Steam\SteamApps\common\XCom UFO Defense\XCOM: Launches the game at least, but has the same graphical glitch of corrupted colors as it does when launching the Windows version from within Steam.