For me, plain black looks very cheap: it shows me that my monitor's "black" actually is grey (hardware dependent of course). By opting for color, the off-blackness is intentional and thus forgivable.
Very hardware dependent. On a lot of modern OLEDs black pixels can be entirely unlit (versus even just the slightest dark gray shade away from black needs to be lit). A lot of the people usually asking for dark modes that use true black like how those pixels look when unlit (on OLEDs). Depending on lighting conditions, things in true black really do disappear from the screen, and it directly cuts down on the overall amount of light emitted from the screen.