The story
Minced form of God's blood. Primarily from the Middle Ages up to the early modern era, and to a rare extent today, people would curse on God's or Christ's body parts rather than breaking the Second Commandment (not to use the Lord's name in vain oaths). In this case, the exclamation refers to Christ's blood shed during the crucifixion, and the Eucharist in the form of wine.
Etymology from Wiktionary, CC BY-SA 4.0.