Hardwood floor discoloration- need some help

So we decided to open up the entry to our kitchen and frame it with reclaimed barn wood. To expand the entry we had to remove some cabinets. Beneath the cabinets the hardwood floor was stained or not stained properly and also shabbily repaired at some point by the previous owners. I guess they figured it wouldn't matter if it was under a cabinet.

My primary concern is the discoloration. Is there any way to stain the discolored areas without sanding down and starting over? There is already uniform finish over everything. Is there a product that will allow me to gradually darken these spots until they match the rest of the floor? Any help is greatly appreciated.

Pics: hardwood discoloration