More specifically, what shade of blue is the earth on average? I want to create an accurate earth-coloured blob in a simulation / presentation.
Well Annan, as you found out, it is a consequence of scattering of incident sunshine, so it is somewhat dependent on atmosphere conditions, sun angle etc. If you look at the color triangle, you will see that it has a straight edge from the red to the mid green, so single wavelength colors in that range can be saturated. But in the blue region, the color chart is quite curved, so even a relatively narrow range of wavelengths in that region, must result in a non-saturated pastel color; so at least you know, that no single blue wavelength can simulate sky blue.
In Physics its called Black body radiation. look at the max energy of outgoing EM from the earth as shown in this image.
As for how Humans views colour is a strange and challenging and different topic, one only has to google optical illusions to see that our brain influences how we see a colour not the input or wavelength.