What about *Do not eat the blood* is so hard to understand.
Jesus did not come to abolish the Law but to fulfill it.
The Law pointed to Jesus. It was not a means by which we become saved. We are not saved by performing certain physical actions.
In John 6 Jesus says it’s the Spirit which gives life, the flesh is NO HELP AT ALL.
Eating flesh in the flesh cannot impart life. Jesus says it doesn’t work.
Look at Psalm 27:1-2, Isaiah 9:18-20, Isaiah 49:26, Micah 3:3, and Revelation 17:6-16: "eating flesh and drinking blood" was symbolic for persecuting or attacking somebody.
So Jesus' listeners knew he wasn't speaking symbolically because He wouldn't have commanded them to persecute Him.
And no disciple decided to leave Him when He said "I am the door" or "I am the vine." They didn't spout nonsense like "You're not made of wood" or "You don't have any leaves." But they DID decide to leave Him when He said (about six times) "eat My Flesh and drink My Blood", "My flesh is true food, and my blood is true drink." [John 6:55]"
His listeners, both those who left and those who stayed, knew this was no mere literary metaphor.
The same with all early Christians. The great Protestant historian of the early Church J. N. D. (John) Kelly, writes: "Eucharistic teaching, it should be understood at the outset, was in general unquestioningly realist, i.e., the consecrated bread and wine were taken to be, and were treated and designated as, the Saviors body and blood" (Early Christian Doctrines.
It as all of them. If you can find me one Christian (not Jews or Muslims: someone who believes Jesus is the Word made Flesh) in the first 500 years AD that denied or refuted the Savior's Body and Blood in the Eucharist, I will be dumbfounded. But go ahead, try.
One cannot understand this with an unaided, unspiritual mind; "the flesh" (your fleshly mind) does not avail to understand that HIS flesh is our saving food. He did not preach that "HIS" flesh is of no avail; that would deny His Incarnation, deny the Eucharistic doctrine of John 6, and deny the power of His bodily Passion, Death, and Resurrection. He cautions us against relying on "the" flesh, that is, our own fleshly minds.
HIS flesh, as He says, gives life.