I can't tell very well in the pic, but is the dark spot more brownish red or actually grey? Hickories have heartwood that is brownish-red and that would be normal, however if it is more grey, that would be bad, indicating possibly dead/diseased rotting wood. I worked with hickory once that had some sort of disease and it had the sapwood that is supposed to be white, which had a pink-ish tint to it. Some of it made a bow and some didn't. The pink sapwood evidently was somewhat bad and weaker.
CP