It's a black sleeve that is bonded into the big grommets on either end.
I don't know what it's made of, or how it's bonded - adhesive or heat?
The sleeve is not as flexible as rubber but it's more flexible than plastic, so maybe vinyl.
In any case, it's not flexible enough, and is almost always split exposing the wires inside.
With broken harnesses where a grommet has been separated, you can see the sleeve inside the grommet like a bushing.