Black gunk covering leaves of my giant bird of paradise

I recently noticed that the giant bird of paradise plants in my front yard along a fence and two lanes from the freeway has black gunk covering the outer most leaves. It does not rinse off. I tried vinegar wash, fruit and vegetable wash, and a harsher kitchen spray. None of these caused the black gunk to wash off. I tried rubbing some of the leaves with the vinegar and water and some of it comes off, but it is very caked on and would take more rubbing than I intend to do. Any similar situations out there?

Any construction on that freeway? I'm guessing it is pollution from the freeway. I worked for CalTrans for 20 years, and many of our freeway plantings looked similar.

Thanks for the comment. I am afraid that if this is freeway dirt, it means I am also breathing it in. I have been reading on particulates that enter the human body when living near a freeway. Yikes!

There is frequently construction. However, I wondered if the drought has made this worse. Additionally, when I looked closely at the leaves, I saw what appeared to be a pattern and also photographed that. today, I found a shiny blue/black hard shell bug on the leaves.

I also see the pattern in your pictures, and assumed it was where you had tried cleaning it and gave up before doing too much damage to the leaves.

You might try different materials to see if anything loosens the gunk without hurting the leaves. Watch the temperature when you are using things. Plants have a harder time with stuff when it is hot.
I would apply a couple of different things- one thing to each leaf- when a cool spell was predicted. Apply in the evening, let it sit overnight, then see if the gunk comes off any easier. Read the label, and apply the strongest dose that is OK for your plant.

Volck Oil
Insecticidal Soap
Dish washing liquid for hand washing dishes, diluted at the rate of 1 tablespoon per 1 gallon of water
Leaf polish
Surfactants or spreader-stickers. Look for names like Water Wet, Water In, and similar names. Some are spray additives that helps the active ingredient stick to the foliage when you are spraying pesticide.
Other?
Perhaps different insecticides or fungicides, because of whatever spreader-sticker or other ingredients they contain. When using a pesticide for its intended purpose the active ingredient needs to stick to the pest. So this same 'stickiness' might make it stick to the black gunk, perhaps loosen it. I would try the other alternatives before trying something toxic.

Thanks for the comments and advice. Our four giant birds are more than 30 years old and have been magnificent until this year, with practically no attention. So, we do not want to lose them. Today, I spoke with the gardener and we investigated the plants further. He looked at the underside of the leaves and found what some online readings had revealed as the potential culprit. It appears to be whitefly. Along the central shaft of the leaves, there are a lot of white splotches which look like the online photos of whitefly. He says he can spray the leaves to kill the whitefly. So, I don't know whether I should try the various cleaning suggestions or just use a pesticide. I hope we are not too late to save our plants.

Picture is not clear.
White flies fly around when you disturb them.
They are difficult to kill.

Black gunk that is not construction or freeway 'stuff':

Several insects create honeydew. It is a type of waste material.
Sooty mold grows on honeydew.

You might notice honeydew as a sticky, clear film on the leaf, and it can drip onto the ground or other things under the plant. Look on garden furniture, cars or whatever is under the plants.
Aphids are notorious for this, and many types of scale produce it. You could easily see aphids or scale on the plants. White flies also produce honey dew.

The black stuff is a fungus that is feeding on the honeydew.
By washing off the honeydew you will remove the sooty mold. You should be able to wash it off with a fairly strong blast from a hose. You could spray with some dish soap: 1 tablespoon per gallon. Let that sit a short while, then wash it off. This dose is safe for plants, might discourage some pests, and can help loosen the honeydew.
By killing the bugs that produce the honeydew you will stop the sooty mold.

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7401.html