What sounds like a self-imprecation is more a blessing -- though those who said it probably didn't realize what they were saying.
Matthew 27.24-25: "When Pilate saw that he could prevail nothing, but that rather a tumult was made, he took water, and washed his hands before the multitude, saying, I am innocent of the blood of this just person: see ye to it. Then answered all the people, and said, His blood be on us, and on our children."
Compare this with Exodus 29.21: "And thou shalt take of the blood that is upon the altar, and of the anointing oil, and sprinkle it upon Aaron, and upon his garments, and upon his sons, and upon the garments of his sons with him: and he shall be hallowed, and his garments, and his sons, and his sons' garments with him."
By the sprinkling of blood, Aaron and his sons were made holy.
So those in the crowd who begged for Jesus' death.
His Blood was on them -- and they were hallowed by it. Hebrews 9.22: "And almost all things are by the law purged with blood; and without shedding of blood is no remission."
(Also worth thinking about is Ezekiel 36.25, a passage showing a pre-incarnation example of Baptism: "Then will I sprinkle clean water upon you, and ye shall be clean: from all your filthiness, and from all your idols, will I cleanse you."