Blasphemy against the Holy Spirit is one step away from judging a brother. A fine and perilous line. Can one blaspheme without knowing it? Yes, if one does not know what blasphemy is.
"Blasphemy against the Holy Spirit is to attribute His operations to the opposite spirit, says Basil the Great. How does one do this? Whenever one sees miracles brought about by the Holy Spirit or any of the other divine gifts in any of his brethren- that is compunction of tears, or humility, or divine knowledge, or word of wisdom from on high (Cf.I Cor. 12:8, Jas. 3:15,17), or anything else that is bestowed on those who love God (Cf. Rom. 8:28) - and says that this comes from deceit of the devil. But he also blasphemes against the Holy Spirit who works in them (Cf.I Cor. 12:11, Eph. 2:2) who says that those who as sons of God are led by the divine Spirit (Cf. Rom. 8:14), and perform the commandments of their God and Father, are being deceived by demons. This is what the Jews of old said against the Son of God... Like unbelievers and those completely uninitiated into the divine mysteries when they hear anything about divine illumination, or of enlightenment of the soul and mind, or of contemplation and freedom from passion. or of humility and tears poured out by the working and grace of the Holy Spirit, straightaway the eyes of their hearts are darkened rather than enlightened... They audaciously aver [def: state or assert to be the case] that these things come from the deceit of demons... To deny that at this present time there are some who love God, and that they have been granted the Holy Spirit... that they have become gods by knowledge... and contemplation, that wholly subverts the Incarnation of our God and Savior Jesus Christ (Tit. 2:13)! It denies the renewal of the image that had been corrupted and put to death, and its return to incorruption and immortality." [Saint Symeon the New Theologian. The Discourses XXXII, 1,2. B#74, pp.335-336]
I have upon occasion trembled at the ease at which brethren blaspheme in this way. On the JTO blog I have experienced this. People whom I have never met- only corresponded with briefly through the comment section or e-mail, step over from doctrinal discussions to blasphemy when they attribute the work and gifts of the Holy Sprint in me to that of demons. This is no exaggeration. I have been denounced by "orthodox" people with clear phrases such as "I proclaim you a false prophet", "you are deceived and need deliverance", you are in prelest and deception", "you are an enemy of the church", and more.
I tremble also when I see in Social Media, the Heretic Hunters who go far beyond the propriety of speaking against heresies, and attack the character and spirituality of individuals, attributing even their good years devoted to the things of God as "false all along", "a demonic plant", or an "agent of the devil". It is a perilous thing to make such statements. It is dangerous, and should it blaspheme- there is no forgiveness. Blaspheming is one step away from judging. We must all watch where we step.