Those are just buds that didn't bud out completely, usually because the plant decided it didn't need them. No problem. Or am I looking at something different?
I did. It's when they're buried and later exposed to sunlight. Necrosis happens; tissue death. And since the tissue is soft in nature, it goes black when it dies. Just like roots do.
If exposed slowly, or in winter conditions, they dry up brownish/regular.