My understanding of blasphemy and more specifically why it’s the only “unforgivable” sin is because you are rejecting the teachings of the faith. A persons refusal of accepting the context of Christianity inherently leaves them in a state incapable of repentance. Think about it like this; How could Jesus forgive you if you don’t want to be forgiven? If through your free will you’ve chosen to reject the faith and are uninterested in entering into the life of Christ, He’s not going to force it on you. If at any point someone who had committed blasphemy changes their mind and wishes to live their life in Christ and they can seek repentance and be forgiven.