That rubber is a seal between the cap and the breather pipe to stop dirt getting into the tank.
If you cant blow into the tank with the cap closed that indicates that the breather hole through the cap is blocked , however you should remember that you can only blow into the tank a certain amount before being unable to blow any further as in fact you are pressurising the tank that pipe is there as a vent to allow the fuel to flow freely.

That makes sense. But why have a breather tube at all if it tank breathes through the cap ??? Oh well