Sounds like a crazy idea, but try Krud Kutter. It’s in the paint section of big box hardware stores. Red and white bottle, cleaner/degreaser. The fact that the wire brush actually worked indicates the discoloration is removable. Do you think the mortar may have been stained intentionally? Or is that grime?

It could be that it’s still wet. Unlike the front walkway, which is exposed to the sun and wind to promote quick drying, that corner needs a little help to dry out. Try placing fans in that area and see if that helps.

Btw, when (date) was the patio power washed and when was it sealed? What detergent and sealer were used, specifically?

Sammy, it was power washed Scott two weeks ago and sealed last week. I had a local handyman do it so I'm not sure what they used.

I’d call the handyman and ask what specific products were used, then look up the instructions online. (I’d be curious to see if the sealer mfr recommends moisture testing prior to application.) I would assume they waited a week between cleaning and sealing to allow everything to dry out. But like I said before, that corner would probably take a lot longer to dry than the walkway, and if it was sealed before it was thoroughly dry, then either a. it’ll take forever to dry and thus lighten up or b. it’ll take forever to dry and the darkness you’re seeing is actually mold, not wetness. Keep in mind these are all just theories on my part. ;)

Do you have before pics, by any chance?

Well, Stan? Any updates?

Sammy, at the hardware store right now. Patio is dry as a bone and the black is still there. Trying to decide what cleaners to try.

Sammy, no luck with any of the cleaners. Here's a pic that will give you an idea of what it looked like before: