Unlike most comic book magicians, Constantine rarely uses magical spells unless he has to, especially in combat. Constantine faces most of his challenges relying primarily on his cunning, his vast knowledge of the occult, manipulation of opponents and allies, and an extensive list of contacts.
Constantine's blood is demonically tainted, initially by a blood transfusion from the demon Nergal, and later by sex with a succubus. His blood has been shown to have healing properties. It also acted as a defense mechanism when attacked by The King of the Vampires in Hellblazer issue 69.
John is a poor physical fighter and has been shown to lose most punch-ups he gets involved in. However, has occasionally won fights, either by using a magical weapon (Hellblazer issue 217) or by fighting dirty (Hellblazer issues 42 and 57, and the graphic novel All His Engines.
Some examples of Constantine's magic:
Divination — Used a pendulum and map to find the location of a magical disturbance. (Hellblazer issues four and 182)
Demon summoning — Summoned the demon Nergal to destroy a monster for him, which it did (although John lost control, due to his inexperience). (Hellblazer issue 11)
Cursing — Placed a curse on his father that caused him to waste away. (Hellblazer issue 31)
Spirit ward creation — Placed a magical sigil on Ellie's soul that prevented the forces of Heaven and Hell from tracking her. (Hellblazer issue 60). As well as using sigils to hide himself from Satan. (Hellblazer issue 85) He also placed various sigils on an abandoned caravan that he housed himself in to hide himself from the demon Nergal. (Hellblazer issue 12)
Golemancy — Raised a golem. (Hellblazer issue 167)
Oclumancy — Erased a man's traumatic memories. (Hellblazer issue 217)
Necromancy — Raised a group of murder victims as zombies to attack their murderer. (Hellblazer issue 230)
Illusion — Making people think he's someone or something else. Or using Illusion to scare susceptible opponents into catatonic insanity. (Hellblazer issue 146)
Constantine has also exhibited considerable mastery in "stage magic skills" - hypnosis, sleight of hand and escapology.