When the great gathering of elves, men, wizards and hobbits occurs at the Council of Elrond, the ring of power is placed in the center of the courtyard, so that those discussing its fate can see how dangerous it is. Unfortunately, for such a tiny and somewhat beautiful object, it has immense powers of persuasion and malice, which immediately prompts a fight between everyone in attendance as to how to dispose of it, with people like Boromir suggesting they use the ring in Gondor as a weapon against the enemy, and Gimli striding right up and trying to destroy it with his axe.
Of course, none of these actions work, and when Boromir gets a little too close to touching the ring, Gandalf stands, chanting in a deep and dark tongue that makes the sky go gray overhead, and all in his presence cower away from him as if in physical pain. Elrond, who almost appears to be nursing a terrible headache immediately afterwards, reprimands Gandalf, saying ‘Never before has any voice uttered the words of that tongue here in Imladris.’ Gandalf does not apologize though, or try to excuse his behavior, because he knows that it proves how serious the situation is, and brings far more gravity to the tiny object in front of them, and that if Sauron wins, the terrible words he has just uttered ‘may yet be heard in every corner of the West’.
But why is it that the black speech of Mordor is considered too dangerous to even be uttered in Rivendell, in all the many thousands of years since Elrond established his stronghold? There are several reasons, some involving the language itself, and some specifically because the ring of power is in front of them at that moment. The first has to do with magic and superstition, and the fear that by uttering the speech in the presence of mortals (who may be weak-willed, as many of the race of Men are), it may corrupt them, tempt them further, or even go so far as to drive them mad.
Similar to the gold lust that comes over dwarves when encountering a hoard that has been long brooded over by a dragon. The one ring knows exactly how to twist itself into people’s minds and hearts, and speaking the language of Mordor in such a pure and natural place is only likely to cause trouble. Similar to the Harry Potter franchise, in which the wizards and witches are afraid of mentioning Voldemort, and refer to him only as ‘He Who Must Not Be Named,’ because they are superstitious that it may bring a horde of Deatheaters down upon them.
Furthering on from this, the second reason is that it is even believed to make elves feel ill, or in pain. When encountering the black speech of the darkest dark lords in existence, elves are thought to suffer, because it reminds them of their history and their painful past, of being tricked by Sauron into taking the rings of power, and of the loss of the gift of the Valar for so many of their race. Although this is more of a fan theory than cannon within Tolkien’s works, it does seem that all of the elves at the council are more affected than the others in attendance, and perhaps explains why Legolas is the only elf to volunteer, because he is the only one strong enough and brave enough to put himself in harm's way, by escorting the ring to Mordor, even though he knows that it may cause him serious poison and injury when it speaks.
This is the third reason, and the one that is arguably the most pressing. Not only is it not wise to speak the language of Mordor in a pure and beautiful place in general, it is especially dangerous with the master ring sat right in front of them. As aforementioned, this is Gandalf’s exact ploy in doing so, he wants to remind the others not to underestimate the evil object or the master who wields it.
Nonetheless, it is a fine line to tread, because uttering the hostile speech so close to the ring definitely stirs and awakens something within it. It is unsure how much Sauron can feel and hear of the ring from such a vast distance, but Elrond is very cautious, because he doesn’t want the enemy being altered to where the ring is, through vicariously recognizing the putrid language uttered in front of the ring. In this way, it is simply wiser not to take risks, and to keep language like that out of Rivendell, and as far away from the ring as possible.