Generally, copper wire should last for quite a long time. I've had stuff sitting in my garage for 10 years or more and I only need to nip off 1 centimetre (if that) to be greeted with pristine shiny copper on the outside of the strands.
What WILL cause problems is when it is being kept in an environment that is less than ideal. This can include - but is not limited to - high moisture conditions and rubbish insulation. Severe age can be a contributing factor, but for the whole 11 feet to be black would concern me.
The fact that the resistance is low indicates it will still perform as a conductor, but termination would be a concern - because even if you did clean it up, I wouldn't feel comfortable unless I checked the connection a few times over a period of time.
Truth is, though, if it looked like that, I wouldn't want to use it.
I'm sure there will be others who can offer more opinions and experiences.....