My refridgerator had a leak and damaged some of the wood floor in my
kitchen.  The floor underneath the fridge was ruined, but also, some
of the floor on the other side of a wall had dark marks underneath it,
presumably from water damage.  What kind of service profession would
be able to fix this and what would they have to do to fix it?  (rip up
the floor?  How much of the floor?)

well i do wooden floors and u would have to rip off all the floor if
it is wet underneath beacause if u dont it will start popping up.. but
if its not all wet then u just have to rip up what is wet.

Stop the leak.remove damaged hardwood.Replace any weak subfloor.Apply
floor leveling compound.Replace with new flooring with the same
thickness,color and texture.As long as joist are not rotten
underneath,this job should go well.A building contractor is the best
bet for hiring to repair.Let me know if you have anymore questions.