Blocker is a period 8 oscillator found by Robert Wainwright. It is a useful dot sparker consisting of two blocks hassling an unnamed object which is a dying spark. It reappears half way through its cycle rotated through 180 degrees.
Blockers can also be infinitely connected by "sharing" blocks, as long as adjacent reactions are no more than one phase apart. They can also be connected with
Coe's p8, which uses blocks as hasslers in a similar manner.
Blockers sharing blocks (click above to open LifeViewer)
Blocker is about the sixteenth most
common naturally-occurring oscillator in Achim Flammenkamp's census, being less common than mazing but more common than jam. On  Catagolue, it is the second most common period 8 oscillator, being less common than the figure eight but more common than Coe's p8.
↑ Achim Flammenkamp (September 7, 2004). "
Most seen natural occurring ash objects in Game of Life". Retrieved on January 15, 2009.
↑ Adam P. Goucher. "
Statistics". Catagolue. Retrieved on October 27, 2018.
15P8.1 at Heinrich Koenig's Game of Life Object Catalogs