Black spots on lid

Brewed a batch of apple cider a month or so ago, and when ready, I bottled it in the 1 gallon containers that it came in. 2 of the 3 gallons came out perfectly fine and delicious. One of them must have continued fermenting and the cap was crusty and yeasty. I would have not thrown it right away if it hadnt smelled like death. My question is, I cleaned the 1 gallon bottle out and the cap, but I noticed that a few of the bottles had black spots on the inside of the cap. Can I clean all 3 of the bottles thoroughly and sanitize them to reuse for my next batch of cider or wine? Also, can widemouth mason jars be used to store cider once it has completed fermentation?

Thanks!

It sounds like mould to me. If I were you I'd bleach the bottles then give them a really good rinse in some hot/boiling water (be careful of the fumes) if you'd like to re-use them. Also, don't underestimate how brilliantly useful actual bottles are, most LHBS sell them pretty cheap (mine does 28 for $28 AUS), and are a lot easier to keep sterile. I don't know about the jars, it seems like an odd way to store it!