What is it to "blaspheme against the Holy Spirit" (3:29)?

Here is what Mark gives. Jesus says, "Most certainly I tell you, all sins of the descendants of man will be forgiven, including their blasphemies with which they may blaspheme" (3:28), "but whoever may blaspheme against the Holy Spirit never has forgiveness, but is subject to eternal condemnation" (3:29). Then Mark adds why he said it: "because they said, “He has an unclean spirit.”" (3:30). Mark doesn't name who "they" are in 3:30. Just before, "The scribes who came down from Jerusalem" (3:22) had said, "He has Beelzebul" (3:22), and "By the prince of the demons he casts out the demons" (3:22), and Jesus's words in 3:28-29 come in his answer, after "He summoned them" (3:23). Matthew (Matthew 12:31-32) and Luke (Luke 12:10) give the saying in their own words. Mark says no more than this, and these notes say no more than Mark: they give his own reason, in 3:30, and leave it there.