Sounds like a stupidly easy question to find out with a quick internet search, but it's not.
I don't want to know the average surface temperature, or the average ocean surface water temperature, or read another article about climate change.
But that's all I found in the past hour.
I'd like to know the average temperature of all molecules that comprise earth, or a best guess scientific estimate.
Black body radiation was my thought as well. It may not be the average including the inner layers, but it's the average at the crust. About -1°F according to Wikipedia.
To add to this, is probably hard because the composition of the interior of the earth is a lot of guesswork. We can only directly observe how much heat is coming out of it.