Binding his foal... — Having declared the spiritual prerogative of Judah, the patriarch now foretells that his land would be so rich in vineyards that the traveller would tie his ass to the vine, as the tree abundant everywhere.
Choice vine is, literally, the vine of Sorek, a kind much valued, as bearing a purple berry, small but luscious, and destitute of stones. The abundance of grapes is next hyperbolically described as so great that their juice would be used like water for the commonest purposes.
Blood of grapes especially refers to the juice of the red kinds, which were more valued in the East than white.