When troubleshooting a customer's problematic atty, I had an MNKE cell fire on a hard short.
Hot button, melted the wrapper on the negative end of the cell, I dumped the battery, let it cool, then put it in my LUC charger and monitored it all the way up to 4.2V.
It now has a pretty translucent orange wrapper to replace the one that melted back, and the cell is back in normal rotation. It's likely that the black marks aren't leaks, or even scorching (if you have enough heat to turn steel black, you've melted the wrapper.) It's probably juice under the wrapper, and when the battery got hot, it darkened, like the last little bit left in a tank is always darker than when you filled it.