I think you're right about the blood flow; it's definitely some kind of circulation issue. My fingers don't get numb; they just feel slightly full or swollen, and it goes away if I loop my thumbs into the loops on my load lifter straps for a couple of minutes, or even just remember to periodically squeeze and release an imaginary ball as I walk.

It's an annoyance, not a problem, and I find using poles eliminates it.