Comment # 4 on bug 104888 from Alex Deucher

I believe setterm uses vesa vbi calls (i.e., calls into the legacy vesa real
mode vbi interface) to blank the screen which only works on boards with a
legacy vbi interface (i.e., won't work UEFI systems).  Doing that goes behind
the driver's back and touches the hw directly.  Additionally, vbi support for
multiple displays is pretty much non existent so even if your board supports
vbi, it's not likely to work if you have multiple displays or are using a
display configuration different from what came up at boot.