Generally OLEDs don't use off for black outside of special-effect low power modes. The display only has stable contrast above some threshold brightness that in a dark viewing environment is going to be noticeably brighter than off, and it looks awful below that. The #000 point is set well brighter than that level.
I don't agree it noticable. My OLED screens all look the same as off when all black in dark rooms. And if you look at specs and display reviews they typically report contrast ratios of 1M:1.
