Church-approved Eucharistic miracles have been shown to be traces of real blood. I recall reading that the blood is always of the same blood type, and that int he Eucharistic miracles in which the host became flesh and was actually subject to examination, it was found to be heart tissue.
On one famous occasion, in Argentina, in an investigation on one such miracle ordered by then-cardinal Bergoglio, the examiner was not told that the tissue he was examining was from a consecrated host that had thus transformed, and to his awe he found that the tissue was behaving not like dead tissue and cells, but like living tissue. There’s a documentary on that matter somewhere.
The Holy Shroud is a whole different story. The marks on it have been studied for decades and have been absolutely impossible to reproduce through any human means. The blood marks are unique to the Passion as described in Scripture. They also perfectly overlap with other similar items such as the Sudarium of Oviedo.
There are also Eucharistic miracles in which the Eucharistic hosts did not change into flesh, but rather remained incorrupt for hundreds of years to this very day. Go figure.
Of course, when the change happens instantaneously, or the host begins to bleed in the priest’s hands, one can hardly attribute it to natural phenomena. But as with all extra-ordinary phenomena, I wouldn’t look too much into it, or try to figure it out.
Here’s a good website to read about Eucharistic miracles:
Vatican International Exhibition
