It's the Holy Spirit's job to speak to a person's heart and move them towards accepting Christ.
Blasphemy against the Holy Spirit means you take what the Holy Spirit is trying to tell you and consider it garbage (wrong, evil, etc.) and reject it.
So, in rejecting the Holy Spirit's message, you're rejecting faith in Christ. And since that faith is what is required to receive forgiveness of sins, rejecting that faith is unforgivable, hence the "unpardonable sin".
It's also only "unpardonable" if you die in that state. Should you change your mind and accept the Holy Spirit's message, and accept faith in Christ, you are no longer "blaspheming the Holy Spirit" and can then receive forgiveness.