Black seed ("Nigella Sativa"), known as קצח in Modern Hebrew, is known for having medicinal qualities - but is it at all mentioned in the Talmud or any Jewish sources as treatment to most diseases?
Ketzach - assuming that this name refers to the same plant in question - is mentioned in the Talmud twice as a cure for heart disease:
"He who is in the habit of taking black cumin will not suffer with pain of the heart. Rabban Simeon b. Gamaliel said: Cumin is one of the sixty deadly poisons, and as for him who sleeps to the east of his store [of cumin], his blood is on his head. [There is no contradiction] because the former refers to when it is baked and the latter when it is not baked. If one eats cumin of the weight of a dinar his heart will be affected; so too if he eats forty eggs, forty nuts or a quarter of a log of honey." (Kallah 3:2)
"Rabbi Ḥama, son of Rabbi Ḥanina, said: One who is accustomed to eat black cumin, a medicine for the heart, will not come to suffer from heart pain. The Gemara raises an objection: Rabban Shimon ben Gamliel says: Black cumin is one of sixty deadly drugs, and therefore one who sleeps to the east of its storage area, where its odor wafts with the westerly wind, responsibility for his blood is on his own head. The Gemara responds: This is not difficult, as this, where Rabban Shimon ben Gamliel said that black cumin is harmful, refers to its odor, whereas this, where Rabbi Ḥama, son of Rabbi Ḥanina, said that it is beneficial for the heart, refers only to its taste. And the Gemara relates: The mother of Rabbi Yirmeya would bake him bread and would stick black cumin to it so its taste would be absorbed, and she would peel it off, so that its odor would not harm him." (Brachot 40a)
However, eating too much will cause heart pain instead:
"BARAITHA. If one eats cumin of the weight of a dinar his heart will be affected; so too if he eats forty nuts, forty eggs or a quarter of a log of honey. GEMARA. R. Joseph said: Only if [he eats them] on an empty stomach." (Kallah 1:20)
(Cumin here is how Sefaria has translated Ketzach)
So it seems that in the Talmud, Ketzach was only in use for specific diseases - those that related to the heart.