Histamine actually serves a number of purposes in the body. But it can also cause problems in excess such as allergic reactions. Nettle and many other substances are natural antihistamines, but not to the point of countering every last bit of histamine. So these natural antihistamines merely help control over expression of histamine.
In fact many foods are naturally antihistamine including yellow onions, watercress, green leafy vegetables, citrus fruits, squash, strawberries, tomatoes, deep cold water fish, etc.
Even without these foods if your adrenals are healthy the epinephrine released from the adrenals will control histamine levels.
So it is not about blocking histamine completely, it is all about regulating proper histamine levels.