That's normal, the base is the fluff of that type of feather. Most breeds regardless of color will have a white at the base of the body feathers.
https://www.hobbyfarms.com/pleasing-plumage-guide-types-markings-chicken-feathers/
I would need to see a feather. Most black birds have white at the base where it's fuzzy I believe. I have had crosses develop whitish hackle feathers, and it can happen in blacks. Young birds that are pure black slowly develop the coloring.
It has the type and skin color of a marans, so it...
Without more definitive information, I would assume it's a mix. Usually people selling purebred chickens would tell you they are purebred, especially if a frizzle.