The Metal Block is a recurring block throughout the Bloons games series. This block is impossible to destroy unless you use a Level Skip. They are highly annoying because they will stop you from reaching poppable bloons. Sometimes, players need to use special power-ups (like the Rubber Block) to get bloons hidden behind them. Sometimes you just need to throw the dart over them to get to the bloons inside.
They appear as metal rectangle blocks with little shading.
They appear as metal, shaded square blocks.
Metal sound in Bloons
Metal sound in Bloons 2 (the sound is quite similar).