Some mods add new Block with their own textures, i am wondering if these textures are independent from Texture Packs one can download which change the vanilla minecraft textures.
ie. If i download a HD Texture Pack and used it, will the new blocks and textures of the mod still show up in their original form? or will the Texture Pack replace them with nothing (even throw up errors) since it has no files for these new non-vanilla textures
Resource packs will only replace the graphics (and presumably sound effects) of the resources they are overriding. Everything else will, in general, default to their vanilla graphics. I have often used an HD vanilla texture pack in a modded environment. The contrast between what is replaced and what is not is not really that big of a deal unless the texture pack is a drastic change from general minecraft appearance.
Texture packs can account for mods if a specific modpack have support for them. Examples of such are Faithful, Soartex Fanver and Sphax PureBD Craft.
As for your question of if the texturepack will show errors or default to their original textures, it solely depends on the texturepack in question. What this means is that if a texturepack create files for the mods but leaves such files textureless, it will show up as such. It will only default to the vanilla textures under circunstance that theres no such file in the texturepack zip to use for the mods.
To simplify things, lets say that vanilla.png represents the textures for vanilla and mod.png represent the texture for mods. If a texturepack have mod.png, it will show the texture even if theres nothing in that file. If no such file exists, it will default to vanilla textures.
