I’m sure there are plenty of guys her who can offer tips on cleaning dark colored siding. The advice I offer is from my experience with dark colored siding.
Specifically regarding dark blue vinyl siding, my nemesis. The siding wears or oxidizes different than other dark colored siding. It takes sometimes days to dry in the shade and while drying the color can look uneven. While drying it can look like there is soap residue, stains or uneven in color.
I always take pictures of the effected areas of blue siding when I do the estimate. Customers don’t pay attention to their siding till after you clean it. Then they see “fading”, “staining” or “residue” in their terms after you clean when it was always there. Pictures have saved me a few times.
Note in the estimate or over the phone that the siding is discolored. Which is normal for blue vinyl, at least in my area. I highly recommend written estimates because customers can’t argue something in writing.
Always pre-wet the blue vinyl or any dark colored vinyl and don’t let your mix dry on it. I know guys say they don’t pre-wet but in the summer heat and with the dark siding I highly recommend it. Just as you would never throw soap on a black car without wetting it first why would you do it on hot dark colored siding. Its a precaution.
I do the same with green and dark green siding but I have always seen blue as the color to wear the quickest.