I don’t think your statement is remotely blasphemous.
Basically, that a human person is a body and soul is essential, and my substance is body and soul. That my body is 5’ 7", pale, and has a red beard is accidental to my substance.
Now this pale, red-bearded body IS me, truly and completely, but that I have this or that physical feature is indeed accidental. The example used above of a Martian with a different set of senses (for example, x-ray vision) is very good at illustrating this, IMO.
To understand why it’s important to assert this, it may help to think of the alternatives given our knowledge of our bodies. If these physical traits, i.e. that this or that color, or even this or that atom, were essential to my substance then I would not be the same person from moment to moment. You see, every second a chemical change takes place in me: some cells die, new oxygen atoms are brought into me and old ones are expelled, my food is broken down and built into new cells to replace the old ones, and the movement of electrons in my brain changes constantly. If these things were the substance, and not the accidents, then I as a continuing being do not exist; I’m just a collection of moments lumped together under an arbitrary heading of “individual”, despite the fact that none of those moments is really the same.
So in every moment my accidents change, but my substance does not. I have new atoms, I literally have new flesh from moment to moment, but who and what I am remains the same. That is why I can speak of myself at all. As one of my Dominican theology teachers put it, if substances were their accidents then we couldn’t even jump in the same river once.
The Mississippi River is not the same water molecules from moment to moment, but it’s the same river. While it’s substance is that of a river, which means that substantially it must be “flowing water”, that it is composed of this or that particular water molecule is accidental. So saying that the water molecules are accidental to the Mississippi River in no way means that the substance of the Mississippi River could be something other than “flowing water”. We could even say that when there were no water molecules, there is no more Mississippi River. Make sense?
What occurs in Transubstantiation is miraculous, because one substance (Christ) takes on a new set of accidents. While they are definitely accidents (the accidents of bread and wine), if they were no longer there then Christ would not be there (which is why we say that Christ is no longer present when the Eucharist spoils and becomes something other than bread and wine). Nothing else in the universe undergoes such a radical change of accidents, in which the substance of one thing is utterly replaced by the substance of another without destroying its accidents.
Hope that helps!
Peace and God bless!