Paul's account of the institution of the Lord's supper mentions the cup, but not the contents of the cup:
For I received from the Lord what I also delivered to you, that the Lord Jesus on the night when he was betrayed took bread, and when he had given thanks, he broke it, and said, “This is my body which is for you. Do this in remembrance of me.” In the same way also he took the cup, after supper, saying, “This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me.” For as often as you eat this bread and drink the cup, you proclaim the Lord's death until he comes.—1st Corinthians 11:23-26 (ESV)
Parallel accounts in Mark, Matthew, and Luke also mention the vessel, but don't specify what might be in it. There's little doubt in my mind, that the cup contained wine and that the early Christians naturally used wine as part of their celebration. But the text doesn't mention the contents at all except that it symbolizes Jesus' blood.
On the other hand, maybe the passage does emphasize the drink, only the drink is blood. That would explain why none of the accounts mention the literal contents of the cup as being wine. My gut reaction to that idea is that it creates more interpretation problems than it solves.
Does the text focus on the vessel and not the drink because they were synonymous or because the contents of the cup were to be downplayed for some reason?