I don't know the specific answers to your questions, but I will say this:
My first Nexus had an issue where the posterized shadows (a result of not enough bit resolution in the color image) were blue. My replacement Nexus (original phone replaced because of cell signal issues) exhibits the same posterization (no display is going to fix insufficient* bit resolution unless interpolation is performed in firmware), but instead of being blue, are a more "natural" dark gray.
Bottom line, there appears to be some variation in the way various Nexuses, for some reason or another, handle shadow detail. One notices shadow posterization, very likely, as a result of the superior (lower) black levels of an OLED compared to an LCD. Why some Nexus displays display these deep shadows as blue, is to my knowledge, unknown.
*Insufficient is relative. If a display has high dynamic range as a result of deepening black levels, then more bit resolution (color depth) will be required of both the source and media transport (your phone's hardware decoders) to prevent an image from posterizing in the shadows. Video is especially susceptible to posterization because I'm guessing most video is downsampled to a lower color depth than stills because of bandwidth considerations. If all videos were 32 bit color and sufficiently large in the XY dimensions, the wireless data transfer rates available to us may not be fast enough to stream the video (as most people do when watching video on phones).