BibTeX:
@incollection{Various,
  author = {Various},
  title = {The tragic death of Mirza Mihdi, “The Purest Branch”},
  booktitle = {Baha'i Stories Blog},
  url = {https://bahaistories.blogspot.com/2015/04/the-tragic-death-of-mirza-mihdi-purest.html},
}

APA:
Various. The tragic death of Mirza Mihdi, “The Purest Branch”. In *Baha'i Stories Blog*.

MLA:
Various. "The tragic death of Mirza Mihdi, “The Purest Branch”." *Baha'i Stories Blog*, n.d..

Chicago:
Various. "The tragic death of Mirza Mihdi, “The Purest Branch”." In *Baha'i Stories Blog*, n.d..