Those learning the history of Katanas will invariably hear the name Masamune, and then likely as not they will hear of Muramasa compared to him, with the legend of the latter as a madman who made cursed bloodthirsty blades.
Well, one might say all blades are bloodthirsty, because that's the point of making them. As it is, Muramasa's work was highly regarded during the final decades of the Sengoku period, and even well into the Edo period. Legend has it that they fell out of favor because two of the people in Tokugawa Ieyasu's clan were killed by them, one deliberately and one by accident, whereupon supposedly Ieyasu banned the ownership of all Muramasas and melted down those he could find --
And yet, the man is said to have owned two Muramasa blades and left them to his family as an heirloom, one of which survives. Was he being a hypocrite? No. Muramasa blades were the most popular katana among the samurai of Tokugawa's home province. They were everywhere. If a Tokugawa family member was going to be killed by a sword it was far more likely to be a Muramasa than anything else. As it is, the story of Ieyasu banning all Muramasa blades is a literary invention, not something to be found in contemporary legal records.
So what exactly happened to all of the Muramasa blades between now and then, and why are there so few? Well, besides the great sword surrender at the end of WWII, during which many fine old blades were lost, the Muramasa blades appear to have fallen out of favor with the Tokugawa clan within a few generations after Ieyasu died. It's not clear why. Perhaps that story about Ieyasu's family losses became the seed of slanderous rumor. Whatever the case, by the mid-1700s the Muramasa had gained their legendary status as cursed blades.
it certainly didn't help that Kabuki theater started to take up the "Cursed Muramasa blade" as a common trope, nor that the blades gained a reputation as being unlucky for the Tokugawa family specifically, such that the people rebelling against the shogunate wanted to use Muramasa blades as a fitting symbol, so those blades became associated with no-good rebels once the Meji period settled everything down.
I imagine what happened was that, as the slander against Muramasa-san developed, and his blades fell out of favor, they were increasingly unlikely to be worn by Edo-period samurai who wanted their swords to be more clearly symbolic of their social class. The Edo period was the era in which the Samurai started getting all romantic and mystical about Katana, long before American japanophiles got involved. So after the mid-1600s a Muramasa blade was the most likely one to be stashed in a forgotten storage room, such that it was neglected for regular maintenance, which would cause the blade to rust into dull scrap in short order. No great loss, right? Muramasa was a madman! Thank goodness we got rid of that cursed thing!
Or perhaps the Muramasa legend led people to destroy his blades deliberately out of fear. Either way, there were few enough real Muramasa blades around in the Bakumatsu period that once they became popular the swordsmiths had to resort to making forgeries.
It is, to my mind, similar to what happened with Sappho's poetry. Not that her work is considered cursed (oh! quite the opposite!) but that she wrote in the Aeolic Greek dialect which had already become archaic by the time of the early Roman empire, so people had a harder time reading her stuff, she fell out of the spotlight, and nobody copied her work from fragile papyrus to sturdy parchment codicies. She suffered an ancient form of file-format obsolescence, similar to what happens with modern computers.
Whether a file format or a poem or a sword, once the makers fall out of favor they begin to fall out of memory, and their works slowly disappear because fewer people have any personal or professional incentive to maintain them.
This is what happened to the Library of Alexandria. By the time it was supposedly burned most of its collection had fallen into unreadable scrap because of budget cuts. And all those ruined castles were ruined because they fell out of use so people took the stones for building material. And so on and so forth. Once a product stops being part of daily life in a culture it starts to fade, and you're lucky if anyone bothers to keep a few examples in good condition.
Or you would be! But archival and museum work is professional these days. Thank your local archivist for keeping weird old junk around. This has been a lengthy endorsement of the professional archival field.