This is probably The Wonderful Story of Henry Sugar, a short story by Roald Dahl. I had not remembered the clot bit but a little online searching found it:
He looks at the place in his chest where the pain is coming from. . .
and he sees. . . or thinks he sees. . . a small dark lump inside the
big vein leading into the heart on the right-hand side. What could a
small dark lump be doing inside the vein? It must be a blockage of
some kind. It must be a clot. A blood-clot!
It is set in the present, although some aspects of that present will now seem a bit dated. Henry does learn to see through cards with practice:
His job now, he told himself, was to keep practising and practising
with the cards until he could see through them instantly. He was
convinced it could be done. Already, on the second go, he had knocked
four seconds off his time. He would give up working with the candle
and concentrate solely upon the cards. He would keep at it day and
And while he used his skill to cheat at cards, in the end, he arrived at a nobler goal:
With the money I make, I will set up an absolutely first-class
orphanage in every country I visit. I will become a Robin Hood. I will
take money from the bookmakers and the gambling proprietors and give it
to the children. Does that sound corny and sentimental? As a dream, it
does. But as a reality, if I can really make it work, it wouldn't be
corny at all, or sentimental. It would be rather tremendous.