~]$ xbacklight -inc 10
No outputs have backlight property

make a file at /etc/X11/xorg.conf.d/20-intel.conf

with the following contents:

#Section "Device"
        #Identifier     "Intel Graphics"
        #Driver         "intel"
        #Option         "AccelMethod" "sna"
        #Option         "TearFree" "true" # causes screen to go black
        #Option         "DRI" "3"
        #Option         "Backlight" "intel_backlight"
#EndSection

I commented everything out because I was having problems where, if I startx with just the basic identifier and driver uncommented with no options, my window manager goes crazy and stops honoring the 30-monitor.conf due to mismatched monitor identifiers DSI-1 vs DSI1.

Before the uncommented lines vs after: basically what looks like a mispelled monitor DSI1 which should be DSI-1.

(Before)
$ xrandr
Screen 0: minimum 320 x 200, current 1280 x 720, maximum 16384 x 16384
DSI-1 connected primary 1280x720+0+0 right (normal left inverted right x axis y axis) 0mm x 0mm
   720x1280      60.08*+
   1280x720      60.00    59.99    59.86    59.74
   960x720       60.00
   928x696       60.05
   896x672       60.01
   1024x576      59.95    59.96    59.90    59.82
   960x600       59.93    60.00
   960x540       59.96    59.99    59.63    59.82
   800x600       60.00    60.32    56.25
   840x525       60.01    59.88
   864x486       59.92    59.57
   700x525       59.98
   800x450       59.95    59.82
   640x512       60.02
   700x450       59.96    59.88
   640x480       60.00    59.94
   720x405       59.51    58.99
   684x384       59.88    59.85
   640x400       59.88    59.98
   640x360       59.86    59.83    59.84    59.32
   512x384       60.00
   512x288       60.00    59.92
   480x270       59.63    59.82
   400x300       60.32    56.34
   432x243       59.92    59.57
   320x240       60.05
   360x202       59.51    59.13
   320x180       59.84    59.32
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)

(After)
Screen 0: minimum 8 x 8, current 720 x 1280, maximum 32767 x 32767
DSI1 connected primary 720x1280+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
   720x1280      60.08*+
   640x480       59.94
   720x405       59.51    58.99
   640x360       59.84    59.32
DP1 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)

Note this "after" xrandr message is only possible to access because X or cwm stopped honoring 30-monitor.conf so it is able to use decent defaults and launch X.
So when X starts, it doesn't honor my 30-monitor.conf and thus my screen is incorrectly rotated/positioned unless I change the monitor to DSI1 from DSI-1.
But if I go in and change my otherwise functional 30-monitor.conf from DSI-1 to DSI1, the next time I startx, cwm (my window manager) has tons of graphical 'artefacts' and practically unusable. Add in the options and the screen goes absolutely black, even though cwm is still running and I can close it.

etc/X11/xorg.conf.d/30-display.conf
Section "Monitor"
        Identifier      "DSI1"
        Option          "Rotate" "right"
        Option          "Primary" "true"
        DisplaySize     75 133 # this is the correct screen DPI
EndSection

But let's say I only enable the backlight option...  just screen artefacts.
It turns out that the "tearfree" option is what makes the screen go completely black.

If I only leave it unconfigured, graphics are fine. But how do I get my xbacklight working?

There are my kernel modules unsupported by linux-libre before anyone asks:

~]$ doas dmesg | grep DEBLOB
doas (shorty@localhost) password:
[    4.238607] iwlwifi 0000:01:00.0: Direct firmware load for /*(DEBLOBBED)*/ failed with error -2
[    4.238624] iwlwifi 0000:01:00.0: Direct firmware load for /*(DEBLOBBED)*/ failed with error -2
[    4.238638] iwlwifi 0000:01:00.0: Direct firmware load for /*(DEBLOBBED)*/ failed with error -2
[    4.238650] iwlwifi 0000:01:00.0: Direct firmware load for /*(DEBLOBBED)*/ failed with error -2
[    4.238664] iwlwifi 0000:01:00.0: Direct firmware load for /*(DEBLOBBED)*/ failed with error -2
[    4.238678] iwlwifi 0000:01:00.0: Direct firmware load for /*(DEBLOBBED)*/ failed with error -2
[    4.238691] iwlwifi 0000:01:00.0: Direct firmware load for /*(DEBLOBBED)*/ failed with error -2
[    4.238705] iwlwifi 0000:01:00.0: Direct firmware load for /*(DEBLOBBED)*/ failed with error -2
[    4.238709] iwlwifi 0000:01:00.0: minimum version required: /*(DEBLOBBED)*/22
[    4.238711] iwlwifi 0000:01:00.0: maximum version supported: /*(DEBLOBBED)*/29
[    4.238712] iwlwifi 0000:01:00.0: check /*(DEBLOBBED)*/
[    4.558494] i915 0000:00:02.0: [drm] Failed to load DMC firmware /*(DEBLOBBED)*/. Disabling runtime power management.
[    4.558495] i915 0000:00:02.0: [drm] DMC firmware homepage: /*(DEBLOBBED)*/
[    4.642568] Bluetooth: hci0: failed to open Intel firmware file: /*(DEBLOBBED)*/ (-2)
[    4.642579] Bluetooth: hci0: failed to open default fw file: /*(DEBLOBBED)*/
[  926.008520] r8169 0000:02:00.0: Unable to load firmware /*(DEBLOBBED)*/ (-2)