Faraaidh of wudhu

Finding a black mark on one's finger after Salaah

Q: I prayed Fajr and was clearing up my room and about 40 minutes later I saw something on the tip of my thumb's nail and some bit inside. So I put it under the tap to test it. The bit inside was gone, but there was still some blackish mark on the tip of my thumb's nail, but not on the skin - I believe. So should I make up that prayer or not? Looks likely to be impermeable, but it's on the tip of the nail anyway. Literally the edge.