Jesus told us of this sin right after being accused of doing miracles by the power of the evil one; as seen in Mt 12: 24-32:
"But when the Pharisees heard it, they said, This fellow doth not cast out devils, but by Beelzebub the prince of the devils. And Jesus knew their thoughts, and said unto them, Every kingdom divided against itself is brought to desolation; and every city or house divided against itself shall not stand: And if Satan cast out Satan, he is divided against himself; how shall then his kingdom stand? And if I by Beelzebub cast out devils, by whom do your children cast them out? therefore they shall be your judges. But if I cast out devils by the Spirit of God, then the kingdom of God is come unto you. Or else how can one enter into a strong man's house, and spoil his goods, except he first bind the strong man? and then he will spoil his house. He that is not with me is against me; and he that gathereth not with me scattereth abroad. Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men. And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come."
Blasphemy is speaking evilly, not stumbling or sinning See Jam 3: 2 NASB, "For we all stumble in many ways. If anyone does not stumble in what he says, he is a perfect man, able to bridle the whole body as well." And again in, 1 Jn 5: 16-17: "If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it. All unrighteousness is sin: and there is a sin not unto death."
The difference between stumbling and falling away is that one gets back up before faith dies out. As seen in Rom 11: 20-24: "Well; because of unbelief they were broken off, and thou standest by faith. Be not highminded, but fear: For if God spared not the natural branches, take heed lest he also spare not thee. Behold therefore the goodness and severity of God: on them which fell, severity; but toward thee, goodness, if thou continue in his goodness: otherwise thou also shalt be cut off. And they also, if they abide not still in unbelief, shall be graffed in: for God is able to graff them in again. For if thou wert cut out of the olive tree which is wild by nature, and wert graffed contrary to nature into a good olive tree: how much more shall these, which be the natural branches, be graffed into their own olive tree?"
Regarding unforgivable sins:
Rev 3: 4 refers to those in the Church in Sardis, who are ready to die, "He that overcometh, the same shall be clothed in white raiment; and I will not blot out his name out of the book of life, but I will confess his name before my Father, and before his angels." And again in Rev 22: 19: "And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book." Add to these the scripture passage above in Rom 11: 20-22.
"But when the Pharisees heard it, they said, This fellow doth not cast out devils, but by Beelzebub the prince of the devils. And Jesus knew their thoughts, and said unto them, Every kingdom divided against itself is brought to desolation; and every city or house divided against itself shall not stand: And if Satan cast out Satan, he is divided against himself; how shall then his kingdom stand? And if I by Beelzebub cast out devils, by whom do your children cast them out? therefore they shall be your judges. But if I cast out devils by the Spirit of God, then the kingdom of God is come unto you. Or else how can one enter into a strong man's house, and spoil his goods, except he first bind the strong man? and then he will spoil his house. He that is not with me is against me; and he that gathereth not with me scattereth abroad. Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men. And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come."
Blasphemy is speaking evilly, not stumbling or sinning See Jam 3: 2 NASB, "For we all stumble in many ways. If anyone does not stumble in what he says, he is a perfect man, able to bridle the whole body as well." And again in, 1 Jn 5: 16-17: "If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it. All unrighteousness is sin: and there is a sin not unto death."
The difference between stumbling and falling away is that one gets back up before faith dies out. As seen in Rom 11: 20-24: "Well; because of unbelief they were broken off, and thou standest by faith. Be not highminded, but fear: For if God spared not the natural branches, take heed lest he also spare not thee. Behold therefore the goodness and severity of God: on them which fell, severity; but toward thee, goodness, if thou continue in his goodness: otherwise thou also shalt be cut off. And they also, if they abide not still in unbelief, shall be graffed in: for God is able to graff them in again. For if thou wert cut out of the olive tree which is wild by nature, and wert graffed contrary to nature into a good olive tree: how much more shall these, which be the natural branches, be graffed into their own olive tree?"
Regarding unforgivable sins:
Rev 3: 4 refers to those in the Church in Sardis, who are ready to die, "He that overcometh, the same shall be clothed in white raiment; and I will not blot out his name out of the book of life, but I will confess his name before my Father, and before his angels." And again in Rev 22: 19: "And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book." Add to these the scripture passage above in Rom 11: 20-22.