Blackthorn is a sentient form of alien plant life that takes on a humanoid appearance who is a member of the Imperial Guard, a multi-ethnic group of super-powered alien beings who act as enforcers of the laws of the Shi'ar Empire. Part of the division of the Guard known as the Borderers, Blackthorn was amongst those Imperial Guard members who allied themselves with the treacherous Shi'ar High Council member Lord Samédàr who was aiding an attempted coup of the Shi'ar throne by Deathbird. Even after the ploy was discovered, Blackthorn, like most of the other Guard members present, still chose to side with Samédàr against the team of superhuman mutant Earthlings known as the X-Men who were defending then-Empress Lilandra.

Later, after Deathbird had successfully usurped the Shi'ar throne, Blackthorn was amongst those Imperial Guard members who clashed with the British team of costumed adventurers known as Excalibur and the team of interstellar freebooters known as the Starjammers over the fate of the then-bearer of the cosmic Phoenix Force, the alternate future daughter of the X-Man Jean Grey named Rachel Summers.