Histamine is involved in the regulation of body fluids. It is released in dehydrated states. Maybe that's got something to do with it
As background: I tend to eat a low salt diet. When I'm in trouble with histamine, salt in water (or just licked off my hand) helps. I don't myself know why, but I've heard that Dr. Batmanghelidj explains why in two books: Your Body's Many Cries for Water, and more, Obesity Cancer and Depression. I haven't read the first in a long time, and don't remember anything there about histamine. So I guess it's in his second book. But I do know his recipe for salt in water helps me.
Without a study at hand, I would assume that sodium chloride per se has no overwhelming antihistaminic properties, since it's already so abundant in the human body and an excessive NaCl consumption has not been linked to reduced allergic reactions etc.
On the other hand, some minerals present in sea salt like selenium and magnesium may have antihistaminic effects.
If you want to be on the safe side: vitamin c might be what you're looking for.
Natural salt helps clear congestion in the lungs and nasal passages. It clears mucus, phlegm, and congested sinuses. Salt is a strong natural antihistamine.
This sounds crazy to me! Salt makes things much worse. Hystamine in fact is released when the body is dehydrated! Drinking a lot of water helps, but definitely without the salt!