After doing some research, the Blasphemy Challenge doesn't really have a connection to an authentic saying of Jesus.
According to Mark, the blasphemy against the Holy Spirit refers to those who claim that Jesus was being controlled by an unclean spirit (look at the context of the passage, Mark 3:20-30). This defense of Jesus was even taught in the Didache (11:7).
This defense looks back on Jesus from the perspective of the later community, which sought to set limits on its ecstatic leaders without inhibiting intrusions of the spirit. Thus, it should not be considered an authentic saying of Jesus.
Mark 3:28-29 and the parallel passages in Luke and Thomas are considered non-authenitc sayings by the Jesus Seminar.
"A man can no more diminish God's glory by refusing to worship Him than a lunatic can put out the sun by scribbling the word, 'darkness' on the walls of his cell." (CS Lewis)
"A young man who wishes to remain a sound atheist cannot be too careful of his reading." (CS Lewis)