When Jesus said the thing about blaspheme against the Holy Spirit, it was in a scene where he had done some healings, and some of the folks who saw it said he had done those healings by the power of Beelzebub. The thing about blaspheme against the holy spirit seemed to be a response to that accusation, so you might say that since Jesus was healing by the power of the Holy Spirit, and they were saying he was healing by the power of Beelzebub, that those folks were calling the Holy Spirit Beelzebub, which was blaspheme. After Jesus said the thing about blaspheme against the Holy Spirit, he reitterated it by saying, “Therefore, whoever speaks a word against the Holy Spriit…” So I get the impression that blaspheme against the Holy Spirit is basically insulting the Holy Spirit, and one way a person might do that is by attributing the Holy Spirit’s work to Beelzebub.