[JRTJH]

Back when function seemed to be more important than looks, slide sides were usually Arctic White, the same as the trailer sides. My 2014 Cougar XLite is that way. The next year (maybe it was 2016) Keystone started using black FILON on the slides. The reason was to mask the black marks from the slide seals and to prevent the "unsightly stains" on the slides. In doing so, they introduced a new problem, heat gain, as you've experienced.

It looks like the appearance (lack of black marks from the slide seals) has "won the battle" since the slide sides remain black, even with the problems in cooling the coach.

Painting the slides may work, but if you deploy your slides frequently, you may wind up scraping the paint off the FILON to be an even bigger problem than the heat gain.

Alternatively, you might consider hanging a piece of solar screen on each end. You could attach it to the slide flange and the side of the trailer, creating a "chimney" to allow for ventilation and shade the black siding on the slide. Possibly others have found ways to help block the sun from their slides and will offer suggestions. But, painting may create more problems than it solves....