IC2 Classic uses a different structure of resourcepack as mentioned in their wiki.

I added the resourcepack (IC2U ResourcePack.zip), provided by the mod dev team, to the directory config/bluemap/resourcepacks. However, it is not used by BlueMap after /bluemap purge overworld, causing missing texture on the online map.

The missing texture is rubber tree.

The first line on the page you linked says:

IC2Classic uses custom JsonModels that load via code, [...]

Which means they don't provide models at all, but generate them at runtime. They only provide textures.
BlueMap needs those json-models to figure out how a block looks and which textures to use. As BlueMap can't just run another mod's code, there is no other way to figure out the block-models.

If you want IC2 Blocks to render, you will need to provide the block-models in form of a normal ResourcePack (in bluemaps resourcepacks folder).

Tl;dr: Adding support for their resourcepack-format won't solve the problem, since it doesn't contain block-models. Closing this issue :)