If one considers, as I do, that all sins, including all sorts of blasphemies, are applicable to the Blood of the Lamb's power of atonement, save this one offense, the blasphemy of the Holy Spirit, then perhaps this is not a sin not categorical sin of the flesh, or 'natural' sin, but as of an elevated level of spiritual significance. If so, then this would collaborate with the additional need of repentance, being a gesture that nobody is able to physically perform except by the spirit, i.e. "...God seeks those who worship in spirit." Afterall, one is hard pressed to worship in spirit unless one 'dies to self.'
I have held thus far, and have yet to be moved from it, that there is (only) one acceptable (freewill) offering of man that pleases God, his faith. And, per God's assurance that, "Their sins and lawless acts I will remember no more" and
"But My righteous one will live by faith;
and if he shrinks back,
I will take no pleasure in him.” Heb10:38
The Holy Spirit testifies of Him, and so if He, the Holy spirit, didn't witness to the pharisees that Jesus is the Son of God when they insisted that Jesus did His work by Beelzebub, which would've have effectively implicated the Holy Spirit to be a liar, then it was only because the Spirit had yet to be empowered to witness (but that is a possibility I've yet to devote more study into).
In conclusion, it is apparent to me that, if denying the testimony (Word) of Son is not forgiven, neither then, now, nor later, then faith in Him is one's only acceptable offer, ever.