What causes black soot on surrounding areas? They run on propane. This just started & haved been used for months.

Burner needs cleaning blocked air ports and jets .

Soot is caused by a few things in gas appliances. Propane even more so. Lack of combustion air is usually the culprit. In propane fired equipment orifice alignment, high pressure or the wrong burner orifice can cause yellow flames the result in soot build up. You need to find the culprit. Best to have a service tech check it out and clean it. when you know what to look for it can be easily fixed.

Normally soot is from a range that hasn't been converted for the gas type, like natural or propane. It may be set up for natural when you're on propane or visa versa. Has this range ever worked properly without sooting?

Yes the burner needs cleaning . Yellow is lack of air causing soot and co2 . turn off do not use . needs a service and to find out why its sooting

You don't have the right Gas to Air ratio..... Not enough air to the burners or you are running to low of gas pressure for propane... Propane takes a smaller orfice or operate than Natural Gas.....
You should have 10" of WC for Propane operation..
You will need a manometer to check it.
The flame should be a blue color with no yellow in it. There should be air shutters right at the orfice to each burners. You will have to play with the adjustment to get the best blue flame with no yellow in the flame.... Hope this helps.....

It shouldn't be burning yellow and emitting soot at all;sounds like on of these possible causes-
(a)blocked burner airways(this can lead to yellow tipping and sooting on LPG powered appliances because insufficient air is getting through to the burner causing it;you can also get this effect on LPG barbecues say if dead insects or spiderwebs are in the burner pipes and airways after a period of storage),
(b)incorrect aeration being set when the burner was converted using LPG jets(too much gas and not enough air causing yellow flames/smells and sooting),
or (c) if you use propane cylinder gas a dicky or faulty regulator. I would get it looked at investigating all the possibilities I mention and taking action to correct this where necessary ie clearing the burner airways(if this is causing your problem), adjusting the aeration in the burners until you get a clear noiseless blue flame without any excessive yellow and sooting(but even after good adjustment you may find still get a very slight yellow tip without sooting at the top of the flame;this does happen with propane gas and there is a slight tendancy for this to happen more often with butane-butane tends to display very slight yellow tips on ignition increasing slightly as the burner warms up to full temperature) or if you use propane cylinder gas replacing the regulator if that is necessary.

Soot is a sign of too much gas and not enough air. Have your propane supplier make proper addjustments.

burners burning yellow , in complete combustion , blocked air ways to burner , it needs a service soon as poss

A clue to what could be wrong is that you mentioned it took 10 minutes for the pilot to go out. It could be your gas valve. Get a technician in to check your gas pressure both before the gas valve and after. Have them check it as the water heater is running.