Anime series about a main character gaining power from blue blob on his arm
You might be looking for Ayakashi (2007, 12 episodes).


  Adapted from the 2005 adults-only superhuman battle action visual novel game Ayakashi. Ayakashi is a parasite life force that gives superpowers to its host, taking in return the host's entire life force. Kusaka Yuu is a student that lost his will to live after the death of a dear childhood friend, until the day a mysterious girl, named Yoake Eimu, appears. From that moment on, his life would never be the same again. Hunted by Ayakashi and their hosts, the power within Yuu awakens and a never ending battle begins.




Yuu KUSAKA in his OP mode.

In episode 2, an evil guy killed many random people in a park and harmed Yuu's classmate in order to provoke him and awaken his powers. He got totally pwned by the enraged protagonist.