Remember kids, if you wanna make blocks go invisible, you don't need to give them textures! Just modify the part of the model that asks for the texture with something like "#missing"
- June (J_n3)

PS: Fluids do need textures tho cause they're hard-coded in the game
PPS: Lava's opacity is hard-coded, it can't be transparent unless you happened to use very specific shaders (like Complementary)