Grady wire was coated, not bare copper. The coating was not as good as tinned, but less expensive.
I've never seen tin corrode.
The coated wire holds up well. If you can bend back and forth what you call black and it doesn't break, it is not corroded. If not, just take some 100 plus grit paper to wire and sand off coating to bright copper if you want to make a connection, then seal it well. For exposure, the shrinks with glue inside are the best. I also use liquid tape at times.
if it was bare copper being exposed wet even for a short period of time, the copper would no longer exist, just a plastic jacket, even well into the length.
There are some sites with quality marrine wire, at rediculous low prices, genuinedealz comes to mind. Including heat shrinks, connectors, etc.