Mercutio's dying words to Romeo and Tybalt, "A plague o' both your houses!" indicate that he blames the feud between the Capulets and Montagues for his death. He does, however, also blame Romeo and Tybalt directly.
As a loyal friend to Romeo, Mercutio defends Romeo's honor by fighting with Tybalt, a Montague. When Romeo tries to intercede, Tybalt deals the fatal blow to Mercutio. Mercutio lays blame on Tybalt by denouncing him and then turns his blame to Romeo. He asks his friend Romeo why he came between himself and Tybalt, saying to him, "I was hurt under your arm."