Interesting, that you are asking a question about a black hose and I'm asking, What black hose?

You have torn into some place that I have not!

The only actual hoses that I'm aware of are from the sunroof drains. They run into the roofs front pillars and and towards the rear door "B" pillars.
There are also two in the trunk under the rear window but they are clear vinyl to yellow viny when aged.

It has always my thinking that the front cowling had some sort lower pan construction that just dumped the water sideways over to the rocker panel area with holes.
This is why I said the most water comes out at the bottom of the "A" pillars. Like just under the rear of the fenders. If the car is tiled up in the front water can go more rearward where there are more outlets that can get plugged as well.

If you take a hose and pour water into the cowl water should come out there first unless they are really plugged! You have to use a hose to flush either way.
You mentioned rust, now you know how it started.

Look for pucker lips. You know, like those you make when you are sneaking up on a hot spoonful of soup! (:-0. (:-)

You will need something small like a coat hanger wire to get up in there but if you have some in there you might have a lot!

Pop the plastic plugs on the inside the car to gain access. It's the only way to assure you have a clean drain trough front to rear.
A small diameter vacuum hose can be pushed in both directions to suck stuff out.


Phil