Blackthorn was a trained Shi'ar Imperial Guardsman skilled in hand-to-hand combat but was not that agile.  He was able to communicate with plant-life and command them to do his bidding.  Being a plant, the Guardsman had no vital internal organs that could be injured in battle and so could survive repeated thrusts by a sword.  There is a possibility that he may have been able to regenerate any lost limbs.  Blackthorn, who had thick spikes on his body that displayed no real offensive/defensive purpose, initially had a stylized long blue shield and spiked weapon but was never shown with them again.