In Zechariah 9:11, we read, "As for you also, because of the blood of my covenant with you, I will set your prisoners free from the waterless pit" (NRSV).
In Does God HAVE To?, I responded to the evangelical emphasis on blood atonement. I wrote the following: "I know that Hebrews 9:22 says that there's no remission of sin without the shedding of blood. But it's interesting that there is a lot of talk about forgiveness in the Hebrew Bible that does not even mention blood or sacrifices. The prophets focus mostly on repentance as the path to atonement--ceasing from evil and instead doing good. There's a lot about blood in other parts of the Hebrew Bible (e.g., Leviticus), but, if every single writer of the Hebrew Bible was as preoccupied with blood atonement as evangelicals, then why doesn't it always come up? It sure comes up in every evangelical discussion of atonement that I have heard!"
After reading Zechariah 9:11, I wonder: Did the Israelites just assume that blood upheld their covenant relationship with God? Although I made an argument from silence in my above statement, I have a problem with that method of argumentation. I've heard professors say, for example, that Sinai had to be an exilic or post-exilic construction because it does not appear in pre-exilic writings. Their assumption seems to be that pre-exilic authors would have mentioned it with frequency if they knew about it. But my problem with this argument is that the pre-exilic authors (e.g., many of the prophets) mention a lot of things incidentally. We wouldn't know that the prophets were aware of Moses, Joseph, Jacob, and Abraham apart from a few stray references, and, even then, they don't give a lot of details about their stories. They most likely assumed that their audience was simply aware of them.
The same may apply to Zechariah 9:11: Faith in the blood of the covenant could have been such a part of Israel's cultural repertoire, that the prophets did not see a need to mention it all of the time. They just assumed it. They did focus on repentance as the path to restoration, however, because they felt that Israel needed a nudge (or, more accurately, a push) to repent.
The blood of the covenant may have been an important factor that guaranteed Israel's eternal survival. In Jeremiah 30:11, God tells Israel that he will make an end of several Gentile nations, while Israel will always exist, regardless of her sin. At the same time, Israel would not get off scott free, for God would chasten her. For the prophets, repentance helped Israel's life run a lot more smoothly, since the result of her sin was exile and devastation. But her obedience was not really what guaranteed her eternal life. What did that was the blood of the covenant, something that the other nations did not possess.
My conclusion is that, even in the prophets, blood may be a significant factor in atonement, in the sense that it secured Israel's life. So evangelicals have a point when they highlight the importance of blood.