Joints between boards have turned black

Q: We purchased a house with blonde hardwoods with an oak border in the kitchen. Around many of the blonde boards, the joints between the wood have turned black, presumably due to dirt. Brushing and scrubbing will not remove it. Any recommendations?

A: It sounds to me more like the finish is de-laminating or chipping off. Exposed wood will eventually turn black. If this is the case the only cure is a full sanding and finishing.