KJV The cup of blessing which we bless, is it not the communion of the blood of Christ? The bread which we break, is it not the communion of the body of Christ?
MGNT τὸ ποτήριον τῆς εὐλογίας ὃ εὐλογοῦμεν οὐχὶ κοινωνία ἐστὶν τοῦ αἵματος τοῦ Χριστοῦ τὸν ἄρτον ὃν κλῶμεν οὐχὶ κοινωνία τοῦ σώματος τοῦ Χριστοῦ ἐστιν
Is the reference to "the cup of blessing" referring to the third cup of the seder, which speaks of the redemption of the Jews from Egypt?
There is a tradition that the [son of] David would pronounce the blessing over the wine of the great eschatological feast in the kingdom of God:
...Death did not put an end to David's greatness and splendor, for he was also among the elect in paradise (Jellinek, "B. H." v. 168; vi. 25, 26), and on the Day of Judgment he will pronounce the blessing over the wine during the great feast (Pes. 119b; compare Cup of Benediction)...
But Jesus is still in negotiations about his drinking the cup (though he actually knows it will not pass unless he drinks it, so he is really "learning [the cost of] obedience"):
NIV Luke 22:
39Jesus went out as usual to the Mount of Olives, and his disciples followed him. 40On reaching the place, he said to them, “Pray that you will not fall into temptation.” 41He withdrew about a stone’s throw beyond them, knelt down and prayed, 42“Father, if you are willing, take this cup from me; yet not my will, but yours be done.” 43An angel from heaven appeared to him and strengthened him. 44And being in anguish, he prayed more earnestly, and his sweat was like drops of blood falling to the ground.
So is he saying that he won't drink the cup of blessing until after his ordeal?
Now, Luke and John seem to disagree on the timing of the killing of the Passover lamb but for Luke, obviously the meal was a Passover meal:
NIV Luke 22:
7Then came the day of Unleavened Bread on which the Passover lamb had to be sacrificed. 8Jesus sent Peter and John, saying, “Go and make preparations for us to eat the Passover.”
9“Where do you want us to prepare for it?” they asked.
10He replied, “As you enter the city, a man carrying a jar of water will meet you. Follow him to the house that he enters, 11and say to the owner of the house, ‘The Teacher asks: Where is the guest room, where I may eat the Passover with my disciples?’ 12He will show you a large room upstairs, all furnished. Make preparations there.”
13They left and found things just as Jesus had told them. So they prepared the Passover.
14When the hour came, Jesus and his apostles reclined at the table. 15And he said to them, “I have eagerly desired to eat this Passover with you before I suffer. 16For I tell you, I will not eat it again until it finds fulfillment in the kingdom of God.”
17After taking the cup, he gave thanks and said, “Take this and divide it among you. 18For I tell you I will not drink again from the fruit of the vine until the kingdom of God comes.”
19And he took bread, gave thanks and broke it, and gave it to them, saying, “This is my body given for you; do this in remembrance of me.”
20In the same way, after the supper he took the cup, saying, “This cup is the new covenant in my blood, which is poured out for you. a 21But the hand of him who is going to betray me is with mine on the table. 22The Son of Man will go as it has been decreed. But woe to that man who betrays him!” 23They began to question among themselves which of them it might be who would do this.
So is it reasonable that "the cup of blessing" is part of the seder and not some new tradition that he started?