Cancer originating from inside the heart itself causing a cardiac tumor are quite rare, a bit less rare are secondary tumors which occur when cancer from a different origination source affects the heart.

I tried to find an exact date and usage origin for ‘black-hearted’ in the context where it would denote a person’s character, and I found several conflicting dates but the earliest date I found states it doesn’t appear until at least the 1630s, almost a hundred years after Katherine’s death. It wouldn’t have been a slight against her character, and it would have been silly if someone tried to. Katherine’s reputation as a goodly, devout, and long-suffering wife and divine queen was firmly established. Even if black-heartedness had meant wickedness in the way it would later, no one would try to pull that stunt on Katherine or besmirch her that way.

Based on the the findings of the embalmer, Katherine’s heart according to him was blackened and had a growth on it which he took to mean that she was poisoned. That wasn’t what Henry and Anne, already getting side-eyed about their whole situation wanted to hear. The idea that Henry and Anne might have poisoned poor Katherine was fuel to the fire as far as making them look bad. So having a black heart made her seem more a victim, not less of one. Given the other symptoms she had as she grew weaker, had abdominal pain, general malaise and nausea, poor appetite, etc…at one point seemed to get slightly better, then grew worse…it made the poisoning theory seem a bit more likely.

Of course she could have just had the misfortune to be one of the rare people to get a primary cancer tumor in her heart, or a secondary. She could have been poisoned after all. There may be a less well known cause or a mistake of the embalmer’s interpretation. It’s been suggested she died of a broken heart. There is an actual ‘broken heart syndrome’ called takotsubo cardiomyopathy which essentially is a weakening of the heart caused by severe emotional and, or physical distress. If she was suffering from a disease that affected her heart, or poisoning, having cardiomyopathy wouldn’t have helped matters. It wouldn’t have caused the blackening of the heart though and isn’t really a fatal issue.

It seems most likely she had a malady of the heart or was poisoned. It might be possible to make a determination if her remains could be tested [I’m not familiar with how cancer shows up in DNA tests of hair/etc..., or how usable an embalmed heart would be], but that’s almost certainly never going to happen either way.