Hmmm I've just done that and I think a black picture will always be slightly brighter than a turned off screen. It's extremely marginal though and if we wanted to be pedantic then we could call it the darkest possible grey, a tint shy of black, but then if we are worrying about it then I think there are perhaps deeper issues none tech related.
I've gone through apps, watching dark YouTube vids, looking at dark pictures and apps with black borders and I can honestly say this is a complete none issue. Modern screens be it iPhone, Samsung, LG, Moto, do not have noticeable issues with this kind of thing. This is nothing but a bun fight.
Thanks for the tip, I think it's given us a conclusion.