It's not a dumb question at all and it's not useful on LCDs. As on LCD screens (TN, VA, IPS, PLS) the backlight is what is consuming most the power, even with a black image the backlight is still on. Hence why blacks look gray on those screens, but on AMOLED screens each pixel is self-illuminating. Therefore any black content will have those pixels turned off and look truly black (Almost anyway, there is some leakage) Using a black image on an LCD screen won't do any damage
If you'd like to read more in-depth about the differences, this article is alright
http://www.androidauthority.com/amoled-vs-lcd-differences-572859/