adding the "particles": "block/cauldron_inner" line made the lever a black and purple cube
Without seeing your file, I'm guessing you forgot a comma. Every texture designation should have a comma after it except the last one.
Commas mean "there's more" so you need one after every texture to tell Minecraft that it also needs to load further textures. The last one is different because, well, it's the last one. There is no more.
In the future, I suggest posting your updated .json file if you have problems like these. Makes it easier for us to spot problems.