Could this be mold on the backs of my cabinets?

The marks alone can’t confirm mold, and color isn’t a reliable way to tell what kind it is. Check that the wood is fully dry and firm, and look for signs of ongoing dampness before deciding what to do.

For a small surface area, clean gently with soap and water or a suitable cleaner used according to its label, then dry it thoroughly. Vinegar fading the marks doesn’t prove the growth is gone, and discoloration can remain even after cleaning. I’d avoid sanding until the wood is dry, since sanding can spread dust.

I’d focus less on the stain and more on whether the basement stays damp. If the panel is dry, solid, and there’s no musty smell or fresh growth, a careful clean and thorough drying may be enough for a small patch. Don’t install it against a wall while it’s still damp.

Bleach isn’t my first choice for porous wood. Whatever cleaner you use, follow the label, ventilate the space, and wear gloves. If the marks come back or the wood feels soft, pause and look for a moisture source before covering it up.

Could also be staining rather than active growth, and a photo or vinegar test won’t settle that. If it’s a small patch and the wood is sound, I wouldn’t jump straight to professional remediation. If it’s widespread, keeps returning, or there’s a persistent damp smell, getting an assessment makes more sense.