Homer listed Nisyros in the Catalogue of Ships bound for the Trojan War. He didn't mention the black beaches or the volcano, but both are still there...
Not all roads lead to Nisyros, which is probably how the islanders like it. But if you're willing to take a 13-hour ferry from Piraeus—the shortcut being a flight to Kos and then a much shorter boat ride—you are in for a genuine Greek island treat and vistas reminiscent of Santorini, just without the inflated prices and sans those French tourists snapping selfies and "forgetting" to leave tips.
Nisyros is not a place where you go for the scene, but rather to tune out. It's located in between Kos, the island much loved by Israeli package tourists, and Tilos, which you've probably never heard of. If you are the lonely brooding type Nisyros could be your perfect match; just consider its founding myth: during a battle between the gods and giants, Poseidon chased the giant Polyvotis all the way from Athens to Kos, where he crushed him by breaking off a piece of his famous trident. A small part of Kos broke away to form Nisyros, whose periodic earthquakes and eruptions are really just the giant's way of giving the middle finger to the god of the sea.
Fortunately the ancient islanders sided with the Athenians in the Peloponnesian War, thus avoiding the kind of wrath that the children of Athena wrought on islands like Milos. Prior to that turbulent period the island had been captured by Artemisia, Queen of Halikarnassos, an ancient Greek city located on the Turkish coast near present-day Bodrum. Though she was an ally of the Persians, Nisyros flourished under canny Artemisia's rule: coins were minted and marble sculptures made; the wine trade was brisk. The first Turkish onslaught came in 1457 and by 1533 the Sublime Porte was firmly in control; in 1912 the Italians came along (you know how they do) and 1948 meant union with modern Greece.
It has been said that Nisyros is an island of wild beauty, that "the monstrous crater that lies at its center and the fascinating succession of colors that come from the white of the houses in the main town, Mandraki, the black beauty of rocks and the enchanting blue hues on the doors and windows of each dwelling create at the very outset an atmosphere of mystery, as though here lies hidden some very ancient secret, a mythical Giant..."
After 13 hours on a possibly overcrowded ferry little Mandraki is a sight for sore eyes: you'll see the precipitous rock rising above the harbor, crowned with the 14th century monastery of Panayia Spiliani. There are ruins of the ancient acropolis and remnants of walls from the fourth century B.C. In the interior mountain villages such as Nikeia and Eborio beckon and then of course there are the three volcanic craters, with steam and sulphur regularly emanating from the ksephysitres, or vents, as the locals call them. Reminiscent of Vulcano in Italy's Aeolian Islands, the volcano is pretty ugly and probably best avoided, unless you would rather inhale fumes from the depths of Hades instead of finding your bliss on a probably uncrowded Nisyrosian beach.
In Neolithic times and likely throughout antiquity Nisyros supplied the rest of the Dodecanese islands with its abundant pumice and obsidian. The beaches still tend to be black and pebbly, such as the fantastic Xoxlakoi (also spelled Chochlaki) and Pachia Ammos, but there are a few sandy ones. But once you go black in Nisyros...