A Blue Bloon is a Bloon that appears in every Bloons game to date, except for Bloons 2 Spring Fling. In BTD series, it traditionally appears on the 3rd or 4th round. This can be altered for player-made levels in the Bloons TD 4 Track Editor if "Light" or "Hard" bloon progression is chosen. It is a "grandchild" of a Yellow Bloon, and it spawns from a Green Bloon; it spawns a Red Bloon. It is only slightly faster than a Red Bloon with a speed of 4 Blocks per second, and has the same speed as a Camouflage Bloon (BTD4). The RBE of a Blue Bloon is 1 (Bloons series) or 2 (other games), meaning it takes 2 hits to pop entirely.
The Blue Bloon is only slightly faster than a Red Bloon, but they are relatively easy to beat, since they only take two hits to fully pop it. They, however, are dangerous in densely packed groups. Such densely packed groups only appear in Bloons TD Battles as bloon sends or in BTD6 in Round 4.