- Prince's of the Apocalypse: Chapter 4, in the Temple of the Black Earth and Temple of Howling Hatred
- Dungeon of the Mad Mage: Level 1 and Level 22
- Explorer's Guide to Wildemount: in the lost magic city stuck in the ice of Eiselcross...
- Descent into Avernus: there's one in Avernus, with a Demon trapped inside.
I've dug a little deeper. There is nothing described as an "obelisk" in the
Tyranny of Dragons modules. There are just those old standing stones outside the hunting lodge that can be used as portals. None of them are black, but they obviously contain magic due to the portals.
The one in the black earth temple in
PotA seems to be the key one in that module. I don't think the ones in the howling hatred temple are related.
The one on level 22 of the
Mad Mage looks to me to be the key one there.
Curse of Strahd doesn't have anything that matches the black obelisks elsewhere. Just all those old megaliths.
The one outside the tomb in
Tomb of Annihilation isn't described as being black, but it is an obelisk, and it radiates magic. It could possibly have been there already, and Acererak just co-opted it for his purposes.
The Nightstone in
SKT is described as being a megalith rather than an obelisk.
There are no megaliths or obelisks of any kind in
Dragon Heist.
I've just looked at
Descent into Avernus, and yes, there's a very obviously magical obelisk in the "Path of the Demons" section. Given what we now know about these obelisks, I'm not entirely sure what the point of having one in Avernus is. How does time work on Avernus? Can you send the hells back in time?
In conclusion, I think it's fair to say that many / most of the official 5e adventures have one, but saying that they "all" do might be a bit of a stretch.