[WIP] Mod Support
If you render a world with custom blocks added by mods, BlueMap does not know how to render those blocks so they will look like this:
If you don't want that, you can try to add and configure the resources of those mods to tell BlueMap how to render those blocks.
If you are looking for a quick fix: Copy the .jar file of the mod that adds those new blocks to BlueMap's resourcepacks folder and reload BlueMap.
See: Installing resource-packs.
You will have to re-render your map and press the clear tile cache-button in BlueMap's menu for changes to take effect.