Spray on TFR/G101/etc
Scrub with brush for a minute then scrub with water on after a minute the water will run clear, then wash window and rinse as normal.
From now on you will never,ever get black runs ( providing the customer keeps to a regular service )
We do this to all our first cleans, it's a build up pollution, soaps, salt etc that has attacked the outer layer of rubber seal, once clean they don't run.
The amount varies from really jet black for a good couple of minutes to almost nothing
And as long as you scrub with cleaner then wash all the black washes away and does not stain the frames
Darran