Blocks are obstacles in the Bloons series. They are generally annoying because they can impede your way but blocks like rubber blocks can be rather useful for aiming at tricky areas sometimes.
Light Brown, darts stop upon contact, 2 hits to break
Very light brown, darts stop upon contact, 1 hit to break
Impossible to break unless using a Super Monkey, dart bounces off
1 hit to disintegrate, dart bounces off
Impossible to break unless using a Super Monkey, darts cannot penetrate it, metallic sound. Note that you still may hear the metallic sound when using a super monkey, if it hits the block indirectly.