If a save is called a "platinum save," it usually means the user already earned the platinum before uploading the save. Sometimes you can earn trophies with it, sometimes it means you can't. Trophies for doing something a certain number of times (like killing a certain number of enemies) are more likely to trigger with a save like that -- whatever the action is, just try doing it one more time. Or go through all the menus and clicking on things and see if something pops. But some trophies, like "story" trophies, are less likely to pop if they have already been earned on a save. But every game is different! Some games, you load up a save and all the trophies pop instantly, or just by doing one quick thing (like in Jedi: Fallen Order, you meditate once and all trophies pop). Other games, you cannot earn more trophies with that save. It all depends. A save like that is often more useful just for the items and other things the character has.
If you're looking specifically for trophies, you'll have an easier time looking for a "save set." Some save sets are only for a few trophies, others are for every trophy in the game. It does mean there are more files to resign. I don't know if there is a save set of any kind on Bloodbourne on NGU.
Another option is to just take a "starter save" and use it to make the game a lot easier. I found one and put a link to it below. I don't know where in the story this is, and because I don't know how the trophy system in Bloodbourne works (and I haven't played it since the week the DLC came out), I don't know if you will be able to earn any trophies with it that have already been earned on this save. But this save says it is infinite health, and one-hit kill. If you are US, this could work for you. If not, see if you can re-region it. If story trophies do not pop with it, then finish the playthrough it is on, go to the next playthrough and see if they pop on that one.
https://nextgenupdate.com/forums/ps4-trophies-game-saves/1030695-bloodborne-op-character-save-dlc-cusa0900a.html