I've noticed that some of my BPs have done that, while others have stayed black. I wonder if it is a quick way for the gate agent to tell who should be boarding at a given time (i.e. if a blue boarding pass appears during group 1 or 2, perhaps the GA knows to send them away)?
My hypothesis is that the BPs expire and turn blue if you open the AA app within some timeframe of the flight departing, thus giving the BP a "chance" to refresh. For flights after which I didn't open the app for some number of hours or days, the respective BPs stayed black and unexpired. I don't have many data points so this remains a very loose hypothesis at this point.
I often check in and get boarding passes for my wife and children as well as myself. Mine are black until they expire (then blue), everyone else who is non-elite is blue. I agree with OP