It seems you might be following some sort of tutorial developed for older versions of Forge. I am not up to date on 1.15.2 Forge modding, but afaik the texture does not need to be specified within the registry of the block (only the unlocalized name). Once the name is set, the game will look for a valid blockstate json file in the resources/assets/<modid>/models/block/ directory. So what you simply need to do there is add a json file which specifies the texture to be used to "minecraft:stone_slab_top" for both the block rendered in the world and the item rendered in the inventory.
I am explicitly trying to make mods for 1.7.10. There are no blockstate jsons in this version. I am not familiar with programming for the newer versions. Any version after 1.7.10 is new. This includes 1.8, 1.9, 1.10, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16.1.
Add "minecraft:" and the texture path if you're putting in the texture where a string goes. If you're doing it where an IIcon variable is needed, get the block's texture. Such as Block.sandstone.getBlockTextureFromSide(2).
Haha I figured it out but this site is so inactive I'm not even sure everyone here deserves to no if no one is online.
This is kind of pretentious. People don't deserve to know since the forums are inactive? Are you insinuating because this forum is inactive and not everyone has hours to dedicate on here that its users don't deserve to be shared knowledge they share for free every day? I'm no moderator, but I don't think that kind of mindset is welcome here.
1. That wasn't a question, it was a statement. 2. My apologies.
My bad, I've misinterpreted the first statement as a question somehow, despite it clearly not being a question, sorry.
Great that you got it solved, and I now see what was wrong, I have no idea how I didn't notice that earlier.