I'm barely starting to learn blender myself! But If you want good tutorials you can find videos on it.

having a specific issue for which I havent found an answer. :/

I've joined two onjects with seperate materials into one object - but when I finish texture painting, save and reload, one of the models turns completely black despite it all being one piece

Did you unrap the uv before or after the parts merging? You might have to set it to use multiple uvs

hmm you could look into the UV maps tab where vertex shapes and blend shapes are located

if there's more than one UV map names they'll conflict each other.