There are two aspects to that:
The coatings (or maybe even choice of optical glasses) will have been made in a way that makes them relatively color-neutral. Do not forget that film had a fixed white balance. Some early lens coatings (single coating era) create a noticeable blue or yellow color cast.
As mentioned in other answers, chromatic aberration will ...
Yes, internally Photoshop converts 16 bit tiffs to 15 bits where 0:32768 is the same range as 0:255 in 8 bits.
Adobe's Chris Cox confirms it here:
And I have verified it by creating 16 bit tiffs in Matlab and examining how Photoshop reads the.
Note that Photoshop expands it back to 16 bits when saving tiff files by ...
Here's a montage of both photos:
Foreground is taken from your first picture, background is taken from the second picture with a gaussian blur (radius 13).
So there's nothing special, just the defocussing.
The stars aren't black; the spaces between them are. (or rather, dark blue)
The brighter bits between that dark ones are the defocussed stars. Due to the defocussing, they have grown in apparent size, overwhelming the blue background. Think of a real star in the sky which is really just a point, but viewed through a defocussed lens appears as a disc.