It’s quite simple, really. The Mass is called an unbloody sacrifice to clarify the fact that we do not behold the Lord’s bloody body as it was seen on Calvary. Instead we behold His body and blood under the appearance of bread and wine. But it is the same sacrifice we behold AND the same body and blood.
I suspect this distinction became necessary as the result of the denial of the Mass being a sacrifice by various Protestant groups.
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