Supposedly there was a prophecy by Elizabeth Ann Seton, an 18th/19th century Catholic saint, about a black box that Americans would have in their home through which the devil would enter. This alleged prophecy is normally used to attack television, which people claim is the black box referred to. But is it true? Did Elizabeth Seton ever actually say this?
Places that make this claim normally do so without any citation, which is suspicious. About the only place I've found online that gave a source was here, which directed me to the work "I See Far" written by "A Friend of Medjugorje" (yes, that is what the author is listed as).
I managed to get a copy of the work and looked at it; it is an anti-television work and uses the alleged prophecy as a piece of evidence for its claims. It was on a different page than described, but the work has had multiple editions and most likely I had a different edition, hence the different page number. Anyway, the quote is there, and its source was page 93 of a 1991 work called "Air Waves from Hell" (another anti-television work) by a priest named Frank Poncelet. I got a copy of that, too, to see if it gives a source. This claim does not come from the author himself. Rather, he includes some of a letter he received from someone (I no longer have the book and cannot double check who it was, but I believe it was identified only as a woman from Boston). This person's letter includes the statement, "In the middle 1800s St. Elizabeth Seton had a vision that in 100 years hence, every American would have a 'black box' in their home through which the devil would enter. Does this sound like TV?"
Unfortunately, they do not say where they heard this or where in Seton's writings we would find this. It is notable, however, that this claim is a bit more specific than what one sees typically online. It says that Seton specifically claimed that in 100 years this would occur. But she died in 1821, and television was hardly in every home in 1921! If this was in fact a prophecy she gave and was of supernatural origin, it could not be television.
However, we have (for now) come to a dead end when it comes to citations. Our ultimate source, rather than any actual writing of Seton, or even someone contemporary to her reporting she made this statement, is simply an unnamed woman from Boston who we have no way to reach to try to find where she got this information.
Is there anywhere we could search through Seton's writings to try to find it? Well, there is a multi-volume set of her writings that is available. But, of course, to look through every single page would be an extremely laborious task. The first volume is available on the Internet Archive here and a search for "box" or similar words seems to turn up nothing relevant. I did find physical copies of the other volumes (it is a total of three volumes, but the third is split into two sub-volumes and thus it's really more like four volumes), but it is obviously impossible to do an electronic search on these physical copies, nor is it feasible for me to look through hundreds of pages to try to find it. I was able to look into the indexes of these books, though, and there is nothing listed for "black," "box," "prediction," or "prophecy."
It's of course possible it was simply left out of the index, so I suppose we can't be fully sure until all of the volumes are digitized, allowing us to do a search through them, and even then it's possible there was a writing she had that wasn't in there (or, alternatively, it was from what someone else wrote about her). But given the inability of anyone to point to any primary source, and the evidence against it being in her writings given the lack of being mentioned in any index, I think we should consider this claim of her making a prophecy about black boxes to be incorrect until someone can demonstrate otherwise. At this point, I consider this prophecy to be inaccurate. Even if it is actually something she said, I don't think people should believe that it is until a source can be demonstrated.
One final note that should be mentioned is that I have seen a few attribute the black box prophecy not to Elizabeth Ann Seton, but rather to Anna Maria Taigi. No sources are ever given for that claim, however, so I have no reason to give it credence.
With the information currently available, I think it is safe to say it is almost certainly an urban legend. Certainly, the burden of proof should be one those who assert this prophecy. Until someone can provide actual proof that this prophecy was given, such as a work by Elizabeth Ann Seton or Anna Maria Taigi (or at least a work by someone contemporary to one of them attesting to the fact she made such a prophecy), I think this prophecy should be considered almost certainly made up. If someone will be able to provide a direct citation to where she said such a thing, I will of course be willing to revise my opinion--but until that time, I consider it to be false.