I agree that the image conjured up by the expression 'washed in the blood' is disgusting. The phrase has a doubtful pedigree (it doesn't appear in the Bible). However, the principle of blood sacrifice has a strong historical basis and survives as a concept because we have no modern metaphor powerful enough to replace the imagery. Christian evangelists' stock example of paying someone else's fine has nothing like the same power to evoke an emotional response.
However, the concept that Jesus put across at his final Passover meal must have been at least as disgusting to the first century Jewish mind as it is to us. Drinking blood seems a 'yucky' idea to us but, to them, it would also be morally, ritually and theologically repugnant. Nothing in our culture prevents us from having black pudding for breakfast (though I don't like it myself) but their laws forbade such dishes. Consuming blood in any form was out of the question and, as for human blood… well! When we read about the words of Jesus in the Upper Chamber we do it with hindsight and with knowledge of Christian doctrine. In their contemporary context those words would be greeted with a shocked silence and some of the disciples may have made a mental note to take the matter up with him at a later date. They never had the chance.
It seems likely that Jesus used the phrasing that has been recalled in communion services over the centuries. John's gospel does not report the words about the bread and wine, but Paul, Matthew, Mark and Luke are consistent enough to suggest a powerful oral tradition. The occasion would have made a deep impression on those who witnessed it; an impression that would be reinforced by the finality of the events that followed. Biblical literalists take it as dogma that Jesus had foreknowledge of the details of the following day. If you forget dogma, the evidence is that he knew this was their final meal together and would have chosen his words carefully to convey a lasting message.
We need to be careful about passing judgement on the attitudes and practises of other cultures and times. Change is not always progress, nor is it a steady progression of improvement. Often it is an oscillation of views, attitudes, moral codes and cultural norms. However, in the matter of blood sacrifice and propitiation, I do see a progression of revelation (or an increasing sophistication of belief). The fact that blood sacrifice is foreign to our culture is a reason why we need a powerful old story to focus our understanding about what Jesus was doing. By linking his personal act of sacrifice to a religious practise that was passing away he gave us a powerful and effective illustration of the meaning of his death. The story of progressive change in the status of blood sacrifice begins much earlier.
Abraham's near-sacrifice of Isaac, his son, was the first stage in the de-brutalisation of the concept of propitiation. The story is so familiar that we forget to ask why Abram never questioned God's command to set out on that ghastly mission. Any fanatic who attempted to murder his son today, in the name of religion, would be quickly locked up and would be vilified by the Press and the people. For Abram to consider the act at all must mean that human sacrifice was an accepted concept in his culture. Though he truly loved his son, he was driven by fear of a god who seemed as brutal as any other god he had come across. This event marks the point where God begins to teach us the difference between him and the capricious, bloodthirsty gods of the ancient religions. By divine coincidence, a ram was provided as a substitute for Isaac. Killing rams may be a nasty way of expressing worship, but it is an advance on murdering much-loved sons. So began the change in our understanding of how God wants to conduct relations with mankind.
The story developed through the Passover experience in Egypt (blood on the doorposts) and then through the Levitical codes and the priesthood, sacrifice became a boringly familiar routine. It became a practical reminder of the principle of substitution that Isaac's release had established. Sacrifice was no longer just a way of appeasing divine anger, but a relief of personal guilt and the beginnings of the concept of soul cleansing. Jesus picked up the theme in the days of the last Jewish generation that could see the practise in action; and he applied the principles to his own soon-to-be-enacted execution. The blood story had come to its concluding chapter, and faith could advance to the position where it could receive the lesson without needing to act it out in grisly practises.
This evolutionary explanation may not appeal to every Evangelical. Songs about the blood are spattered throughout Protestant hymn books and the expression 'washed in the blood' is liberally used in sermons, prayers and even in conversation. My view is that it is a revolting piece of misplaced evangelical jargon, which lacks scriptural validation and presents a theology that the gospel replaced. Repeated sacrifices belong to the Old Covenant. The New Covenant talks of blood for redemption, purchase and sealing - all of them being once-for-all concepts that nullify the need to repeat the nasty business. Whoever wrote Hebrews says that the blood of bulls and goats could never take away sins, but Jesus dealt with the problem once for all, so it is time to stop the blood talk.
I think it is as appropriate to keep talking about blood as it would be to celebrate our past success with nappy training! The cleaning up of our act was a necessary part of our development in both cases; but it was in our past. Once we have received God's grace the work of spiritual cleansing does not have to be repeated. When Jesus washed the apostles' feet, Peter said "not just my feet but my hands and my head"… Jesus replied that, for the person who has already had a bath, all that is needed is to remove the street dirt that has got through the sandals. The blood of violence and injustice brought me into salvation, but the gentleness and reliability of grace keeps me from plunging back into sin.
Our thinking has been framed by stories from human history that we would prefer not to repeat. Kings were violent and got away with it; wars were fought over trivial issues; torture and inhumane execution was considered normal. These things are part of our heritage but we hope are no longer factors in our culture today. Blood sacrifice fits into that pattern. Levitical ritual was barbaric, and so was crucifixion. The Jewish Temple was a public abattoir, but the horror is past and we don't need to keep harping on about it or pretending it was 'nice'. Evangelical talk of 'the blood' can be as glibly unthinking as references to eternal punishment. The ease with which some people talk of these things shows that they are too familiar with the jargon, but have forgotten the horror of what it really means.