As to what 'blasphemy' means: It's from the Greek βλασφημία, literally 'injurious speaking', and could apply to speaking about humans (defamation) or the gods. As for what it means in practice, these days nothing. The English, who have an established religion, still had trials for blasphemy into the 1990s but abolished the offense in 2008.
As for the US,
Wikipedia will tell you:
The Blasphemy Act 1697 (9 Will 3 c 35) was an Act of the Parliament of England. It made it an offence for any person, educated in or having made profession of the Christian religion, by writing, preaching, teaching or advised speaking, to deny the Holy Trinity, to claim there is more than one god, to deny the truth of Christianity and to deny the Bible as divine authority.
This became part of the received law of English colonies, including the North American colonies, up to the time when having obtained their own legislative powers they chose to change it. The First Amendment (1791) to the US constitution replaced it at the Federal level. The Due Process part of the 14th Amendment (1868) has been interpreted in the 20th century to extend the religious part of the First Amendment to the states. (Although the offense of blasphemy is still on the books in several states, these days a prosecution could be successfully defended.)
As for the Holy Ghost, he or she or it is part of the Trinity. The doctrine of the Trinity is not supported in the NT; for instance, in all four gospels Jesus denies (in direct speech) that he's God, and there's no suggestion that the Holy Ghost is anything but God's messenger or errand boy. (The Holy Ghost is descended from the Jewish 'ruach' or breath of Yahweh, but as I understand it, the ruach is a manifestation of Yahweh himself, not a distinct agent.)
A further problem with the Trinity doctrine is that it's incoherent: it claims that God is a single being with three persons (Father / Yahweh, Son / Jesus, and Holy Ghost) but that each of those persons is individually 100% of God as well. A moment's consideration will show you that 100%+(a completely distinct) 100% + (a completely distinct) 100% = 300% = three gods. And if you consider that God is 100%, then each of his equal constituents can only be 33.333% of God, and would presumably form a board of directors where the majority vote prevails. There are no other important options. The RCC and the Anglican / Pisco churches (and I assume all the other Trinitarian churches capable of clear thought) attempt to finesse the problem by calling it 'a mystery in the strict sense' and to declare that it is not contrary to reason but above reason: you will instantly recognize that as meaningless nonsense, there being no test which can distinguish what is 'above reason' from what is contrary to reason.
So to blaspheme the Holy Ghost must mean to speak contemptuously of God. If you speak contemptuously of Jesus you're not speaking about God unless you're a Trinitarian, in which case you're in a zone where reason has been abandoned. If that's where you are, I respectfully suggest you find your way to the door.
As to whether blasphemy of God can be forgiven, it depends whether you think God is an irrational, irascible, insecure being who can only thrive on constant grovelling (think Trump), or who understands all and therefore forgives all, and does so with even more pleasure if asked. Such a being would never acknowledge a word like 'unforgivable'.