"blocks" refers to both a song composed by C418, and a music disc that plays the song when inserted in a jukebox. The music disc is obtained from creepers killed by skeletons.
Because TNT ignited by a flaming arrow attributes all resulting kills to the entity that fired the arrow, a skeleton igniting a TNT block due to holding a bow enchanted with Flame, or shooting through lava or fire, also causes any creepers killed in the explosion to drop a disc.[1][2]
Previews of tracks in Minecraft have been shortened to 30 seconds on this wiki in accordance with fair use rationale. "11" and "calm4.ogg" are exempt from this.
"blocks" is an upbeat chiptune-style piece with a shuffling waltz rhythm. Near the end, the song switches to a slow, dark undertone tune.
Music disc "blocks" now uses the .ogg format. Before, "cat" and "13" were the only discs in the .ogg format; all the other discs were in the .mus format, which was decrypted by Minecraft on-the-fly.
With the introduction of jukebox song definition, music disc "blocks" now has an associated .json jukebox song definition under data/minecraft/jukebox_song