I felt that this warranted its own thread rather than a post in the Small Edits thread as this matter concerns not only an item description page, but also multiple Gen 4 and Gen 5 analyses.
Analyses have long claimed that Leftovers is superior to Black Sludge on Poison-types.
Originally Posted by Fat Item description
Most of the time, Black Sludge will be inferior to Leftovers. Why is this, I hear you ask? Surely Black Sludge is superior if held by a Poison-type, as not only does it heal the same amount, but it also damages any Pokemon who tries to use Trick? Well, yes, this is true. However, there is a problem with this - even if your opponent does accidentally get Black Sludge by Trick or Switcheroo, your Pokemon is still crippled, while the Trick user can just shrug and Trick your Black Sludge onto a different Pokemon on your team, later in the battle, thus crippling two Pokemon on your team, instead of one. Of course, you could always predict this and switch in your original Poison-type - but this is a gamble. In short, Leftovers is the safer item, even for Poison-types, but if you want to gamble with Black Sludge, they will be functionally identical in nearly all cases.
These claims are not true in practice. Switching in your original Poison-type should not be a gamble if you were willing to switch it in to begin with: it would likely have been a counter or check to the user of Trick in the first place. Furthermore, there are other factors to consider: Black Sludge will wear down its holder (absence of Leftovers plus residual damage), so if you can maintain momentum properly, the Trick user might not even have an opportunity to switch in safely and Trick the sludge back. There's also the fact that the Trick user has to waste a turn to do this, still risking the original Sludge holder / death fodder switching in.
I started using Black Sludge as soon as I realized that it was a better option. I also have noticed that several good players have started / had been using it, as it has come into play in handful of my matches (where I was the one using Trick and getting fucked over by Black Sludge).
(By the way, I dislike this description's tone, especially given the fact that I believe it is wrong.)