When I color pick my pasteboard, Photoshop gives me a completely different color value.

There seems to be some confusion over what my problem is, so I'm adding a third image to try to explain (see below). 

I know I'm probably design-splaining, but just to be clear on terms, the pasteboard is the dark grey background of the application, not the canvas background. It's a nonprintable area, but in the past, I've still been able to color grab from it to get the exact same grey. I use that dark grey to make layers that block off the spine and back cover of a book design so I can focus on the front exclusively. If everything goes smoothly, it results in the OP's first photo: a dark grey rectangle and bleed area that looks identical to the Adobe pasteboard behind it, giving the illusion that I'm only working on a front cover canvas.

However, today when I tried to color pick the pasteboard, I got a light grey that isn't remotely close to the color of the pasteboard. See the photo below. The ring is my eyedropper tool picking the color of the pasteboard, but it's picking a light grey, not a dark grey.

I've been designing professionally this way for literally years, so I'm not crazy, even though right now I feel crazy. But the color picker should be picking up a dark grey with a hex of #282728. That is the hex color of the pasteboard. But instead, the color picker is sampling the pasteboard and picking a hex #888888 instead. Everything I color pick directly on the canvas is accurate, but not the color of the pasteboard.

Not sure I understand the problem - according to the Layers panel, the light color is the top layer in the lower screenshot? Your color picker is set to "sample all layers", so that checks out too.

There may be something wrong here, but your screenshots seem perfectly fine?