I have seen these odd looking grayish patterns on a number of boards. Wood is pre-finished 5-inch solid hickory by Virginia Mill Works - LL house brand. Wood looks ok other than this. If I turn some of the boards over, it looks like it is in the wood itself. This is supposed to be a rustic, distressed type of flooring with a wire-brushed matte finish, but I have not seen this before. Is this something to be expected with hickory?

This looks like a "sticker" stain that can happen when the wood is stacked as it is drying. It is usually a surface discoloration as you can see where the milling has removed all of the color in the channels on the bottom of the sample board.
You may pay more for this type of character marks.

I don't get it.

Maybe, but a lot of the stains appear to go all the way through the board. Wouldn't a sticker stain be more on the surface of the board?

Could it possibly be some kind of spalting?

https://wunderwoods.wordpress.com/2011/09/13/white-wood-sap-wood-and-spalted-wood/

I understand that Hickory can have a lot of patterns and variation of colors, and that is fine. However, if this is in fact a defect in the flooring, I am going to pull most of these boards aside and send them back. I could also put some of them in areas that will be covered by a rug or furniture, I suppose. There are quite a few boards that have marks like this, although this is one of the worst that I have seen.

Spalting is from the wood fibers being attacked by fungus. The edge of the spalti ng will have a black line as a boundary. The spalted areas will have less density.
One characteristic of Hickory are brown colors from heartwood.
When wood is dried to less than 19% moisture content any fungus goes into suspended animation and stops growing.
If you don't like the color variation you could get extra and send the rejects back if you re-package carefully, I suppose, call to make sure. Sometimes distributors will charge a re-stocking fee.

Interesting comments Pete. I'd just put 'em on the closets

http://www.woodweb.com/knowledge_base/D ... Alder.html

They are ordering some more boxes so I can exchange any that are defective, so hopefully I won't find too many more of them.

I did several searches trying to find a picture of wood with "sticker stain", and had no luck. Found a couple of articles that discuss it, but no pictures.

I guess at the end of the day, if this is just part of the natural "character" of hickory, I can live with it to an extent. But if this is in fact a defect, I don't want to pay for it, and I sure don't want to install it. I have looked at dozens of pictures of hickory flooring, and I have not seen any pictures of boards that look like this. Has anyone seen anything like this before?