Calvin was involved in the prosecution of Michael Servetus for "horrible and execrable" blasphemies which included, amongst other things, likening the Holy Trinity to a three-headed dog. This led to Servetus being slow-burned to death at the stake in 1553 with Calvin's approval.
Calvin, like most of his contemporaries and predecessors,regarded false teaching as blasphemy. The word comes from the Greek meaning harmful speech. Traditionally it was considered a very serious crime because it could cause others to lose salvation. This did not, of course, fit with predestination and Calvin saw punishing blasphemy more as a means of vindicating God's honour, and preserving the peace.
St Patrick is said to have used a shamrock, a three-leafed plant, to demonstrate the concept of the Trinity ; this is why it appears on the new UK one pound coins. My Sunday School teacher used a block of Neapolitan ice cream (chocolate, vanilla and strawberry), which we then ate with no idea of sacrilege. These sort of analogies are, as Michael16 points out, not intended to be the last word on the nature of God, but may help impart a first idea.
They could be pushed too far. If the idea that God resembled an egg were to lead on to the revelation that His name is Humpty Dumpty, or that people should give up on Easter Communion and eat chocolate eggs instead, then that might be pushing closer to blasphemy.
Words can change their meaning somewhat and blasphemy is, these days, more associated with mocking , flippant and scurrilous abuse of sacred things. Even with Servetus, part of Calvin's objection was his mocking tone. Teaching false doctrine, in a polite and serious manner, is not regarded as blasphemy now.
The Free Church of Scotland website has an article from former Moderator, John Ross, quoting a legal opinion on blasphemy in Scottish law as involving railing or scoffing. Merely stating, or arguing for, heretical opinions is not regarded as blasphemy, in the current meaning of the term. It is certainly true that, as OP says ,there are several people (at least several) who don't understand the doctrine of the Trinity, but they would not be regarded as blaspheming by saying so, or by using an analogy similar to that used by St Patrick.