A guy on another forum posted this. A dark line as you can see and opened to a very shallow depth where the grip cap would be. What exactly is this and what are the potential problems if any down the road? Thanks.

I'm not sure what that is, but most of the time when you see concentrated black areas in walnut and other woods it is due to some type of metal, wire being one of the more common causes.

If the crack is superficial it can be filled with epoxy, colored apprpriately, and then capped with a metal cap and it will not be an issue. However, if it is deeper this may be very problematic. It appears as if the mineral line where the crack is located extends all the way to the lower tang. I cannot be certain, but it may cause more problems, particularly if this is a heave load rifle. I do not proclaim expertise, but have had some experience with cracks and voids and try to avoid any problems in the wrist and recoil areas.

That type of defect is most common in cherry.I don't remember the proper name but it is a weakened area.It isn't clear how extensive it is from the photo .I'd take a carefull look and consider exactly how it runs and the carridge etc.

I should like to know if this dark line extends from the trigger guard through the grip cap and beyond to the wrist area ?. Can you see this line if you are viewing the top side of the stock ?.

As this appears to be what I can see by the photo . Next what caliber rifle is this ?.

It could be a number of wood defects , including pitch pocket ,black check ,gum spot or ring distortion . All of which were caused by insects grubs weevils maggots an such while the tree was living .
It could be a mineral streak or bluing ,the latter which is caused by a fungus following an insect wound in a living tree .

Unfortunately I can't tell from the photo .

My area of concern would focus around the trigger guard screw hole , which again appears to be in close proximity to the line .

The grip cap area is easy enough to conceal any epoxy repair . How deep does this gum vein appear to go in the grip cap area ?.

The reason I asked about caliber , if this is a light recoil weapon , then more than likely no harm no foul will occur . If we are speaking of a Magnum or large recoil round , then that may alter the extent of repair procedure I would under take if it were mine .

Shoot Straight Know Your Target . ...

Isn't that on a double barrel? Shotgun?

It's a pitch pocket.

It runs thru the grip and up towards the toe dying out as it goes. It's a old stevens model 335 12 gauge. He recieved it as a beater and is looking to give her some more life. Not a lot invested other than time and labor. The pitch pocket makes the most sense to me as well.

It looks to me it has wind shake,I would think that ,that wood be the weak link to the whole stock and will crack. Just my two cents

That looks like the pitch pockets that are common in cherry. That's my vote.