In the fourth Gospel we find that Jesus says that we must eat his blood and his flesh so as to live. However, the author (or one of the authors) of the Fourth Gospel invented Jesus' words in order to fulfill a theological agenda against the Jews who were opposing Christians in the beginning of the second century. So, it is unlikely that Jesus said "you must eat my blood" or "before Abraham I AM", as the Fourth Gospel tells us.
Nevertheless, the words "unless you eat the flesh of the Son of Man and drink his blood, you have no life in you", are in the Fourth Gospel, so, if a christian believes that Jesus said that, it clearly contradicts the way Jehovah's Witnesses celebrate the Last Supper and in certain sense contradicts the ban of eating blood.