If you are interested, here is what the NICNT-Matthew commentary (R. T. France, p.482-3) posits about Matthew 12:31, 32, which is the parallel account to Mark 3:29:
31-32 The saying about an "unforgivable sin" has often been inappropriately, and sometimes disastrously, applied to contexts which have little to do with its original setting. As it appears here in Matthew, it is specifically concerned with what the Pharisees have just said. In 9:3 the scribes had accused Jesus of blasphemy; now the charge is returned. . . The term [blasphemy] could also be used in a less technical sense for "slander" of fellow human beings (Mt 27:39; Lu 23:39; Rom 3:8; etc.), and the use of "speak against" as a synonym for "blaspheme" in v.32 reflects that usage, but here the reference to the Holy Spirit as the object of blasphemy requires the full religious sense of the term. The opening "therefore" [Beginning of verse 31; NWT has "On this account" - Bobcat] indicates that in this context blasphemy against the Holy Spirit is to be understood in terms of the Pharisees charge in v. 24 [of Mt 12], attributing what is in fact the work of God's Spirit (v. 28) to his ultimate enemy, Satan. It is thus a complete perversion of spiritual values, revealing a decisive choice of the wrong side in the battle between good and evil, between God and Satan. It is this which has shown these Pharisees to be decisively "against" Jesus (v. 30). And it is this diametrical opposition to the good purpose of God which is ultimately unforgivable. The point needs to be emphasized, since the language of this saying has sometimes been incautiously applied to real or supposed offenses "against the Holy Spirit" 32 which have nothing to do with the blasphemy of these Pharisees, and serious pastoral damage has thus been caused. 33 This saying is a wake-up call to the arrogant, not a bogey to frighten those of tender conscience.
. . .
The balance of the clauses in v. 31 requires the first to be read as a foil to the second, not as a declaration in its own right. It is beside the point to question whether any worse sin could be imagined; the point is that blasphemy against the Holy Spirit stands out from the run of "ordinary" sins as being uniquely serious. It is to declare oneself against God. It is to "call evil good and good evil." (Isa 5:20
Footnote 32 says:
Did. 11:7 [Didache - see here] applies a version of this saying, taken out of its gospel context, to those who question the authority of a prophet speaking "in the Spirit."
Footnote 33 says:
Among many examples, see the graphic account by George Borrow in Lavengro, chs. LXXII-LXXVII, of a Welsh preacher he met on his travels who believed that as a child he had committed pechod Ysbryd Glan, "the sin against the Holy Spirit," and was now irrevocably damned. See also John Bunyan's account of his wrestling with this issue in Grace Abounding 147ff.
[Material in brackets is Bobcat's, for clarification. I hope I got the links to the several books correct.]
Hope this is useful.
Take Care