This is Sniper’s right thumb I’ve seen in various sfm photos. Did he hurt his thumb or something? His thumbnail is all black but the rest of his fingers/nails are fine looking.
His sniper rifle has a manually operated bolt action on the right-hand side. The bruise (a subungual haematoma) is caused by trying to reload too fast (too many people wanting him dead on the battlefield?) and getting his thumb caught in the sliding mechanism.
(Gif by Coverop.)
Watch how his right hand moves to pull back the handle, unload the spent cartridge case, and slide the fresh round into the breech:
(Gif by Payface.)
The bolt-action mechanism looks something like this (source: wikipedia) - it is allegedly quite painful to get your fingers pinched in it:
That’s Valve for you - completely ridiculous canon, perfectly detailed character designs.