Black gunk builds up under the nail sheath.
You will see gunk in 2 areas:
1. at the start of nail bed
2. under the skin fold
You can wipe it off with bath wipes or soft tissue wrapped around your finger. Q-tips don't really work well because the tips are too soft & round.
Be careful not to be too rough near the nail bed...if possible, go close to it without touching it.
To clean under the skin fold, hold the skin back with your one hand and clean with the other.
Once you clean them, it may last a week or more until the next cleaning.