block.getType(); // From org.bukkit.block.Block

// Our Bukkit Block from wherever (in my case from event#getClickedBlock() in the player interact event)
Block block = event.getClickedBlock();
// Get the CraftBlock from the org.bukkit.block.Block
CraftBlock craftBlock = (CraftBlock) block;
// Get the NMS Block from the CraftBlock
net.minecraft.server.v1_13_R2.Block nmsBlock = craftBlock.getNMS().getBlock();

// Get the key of the block (minecraft ID)
String key = IRegistry.BLOCK.getKey(nmsBlock).getKey().toUpperCase(Locale.ROOT);
System.out.println(key); // Yields "DEAD_TUBE_CORAL_BLOCK" as it should

// Standard Bukkit API method
Material m0 = block.getType();
System.out.println(m0); // Yields "LEGACY_AIR"

// Mimicking the "getType()" method from block with the nms block we created
Material m1 = CraftMagicNumbers.getMaterial(nmsBlock);
System.out.println(m0); // Yields "LEGACY_AIR"

// Directly accessing the hash map "BLOCK_MATERIAL" in CraftMagicNumbers
Field field = CraftMagicNumbers.class.getDeclaredField("BLOCK_MATERIAL");
field.setAccessible(true);
Map<net.minecraft.server.v1_13_R2.Block, Material> map = (Map) field.get(null);

// Simple retrieval from the map using the NMS block we created above
Material m2 = map.get(nmsBlock);
System.out.println(m2); // Yields "DEAD_TUBE_CORAL_BLOCK" as it should??