He COULD NOT have demanded that people break the very Law He came to fulfill. That would have been an occasion of sin for Him.
Jesus came to FULFILL the Law, not abolish it, so He couldn't have told the disciples to eat real blood. It had to be symbolic, as the Passover meal was.
Peter said in Acts 10;14 when he had the vision of the sheet and unclean animals.... But Peter said, By no means, Lord; for I have never eaten anything that is common or unclean.
That would include eating blood, which would have made him unclean for the Passover if it was real blood at the Last Supper. As an observant Jew, he would never have eaten blood or anything he thought had blood in it.
Or he lied to God about never having eaten anything unclean.
Thats why so many left because they thought, like the Catholics do, that it was the literal, physical flesh and blood. The apostles knew better and stayed.