I kind of wonder if your beef heart is dry aging with the carcass. When you hang a beef in the cooler usually the organs hang with it until it is time to process the beef- it could be the time it's sitting in the cooler. I am not sure how all slaughter houses do it- but you might be able to ask for it to be packaged right after it is slaughtered. If the slaughter house has unstable temperatures- that can change the meat too. Not to mention- Slaughter facilities have a sort of smell to them that the hearts might be taking up a little (like where it hangs in the cooler might have something to do with it).

I haven't noticed it with the hearts I've eaten yet, but I got a cow tongue once that smelled like a stinky barn. After slow-cooking it was fine, but it did kinda turn me off from buying tongue for a bit.

I like to trust my nose when it comes to food, so eating something that smelled like it was marinated in manure and then rinsed off before packaging doesn't seem quite right. That's probably just the smell of a cow's mouth, but...eh.

I eat a lot of heart, too. It comes already sliced for $1.99/lb where I get it, and I've probably had it a dozen times now. It's conventional, but I just trim any fat the butcher might have missed. I prefer to cook it medium-rare like a steak in the cast iron pan. Good stuff!

The last time I had heart, I used it to make liver pate. The trick to both the liver and the heart is not to overcook them. Think lightly seared, very rare on the inside.

I soaked both in a bowl of water for about 20 minutes to let the blood and other stuff out of it, and rinsed in the middle, then when ready, removed all the silver skin off them. The liver had a very light smell to it, almost like grape fruit, and I was very tempted to try to eat it raw. Maybe one day I'll work up the courage to do so. :)

I used this recipe here: http://farmlet.co.nz/?p=199 using both the heart and liver instead of just liver and ghee instead of butter. I probably used a lot more ghee, garlic, and wine than the recipe calls for.

Once the sauce reduces down a bit, I add the heart and liver, and I don't let it cook for very long, just let it get lightly coated in the sauce and cook it only for a few minutes on all sides before killing the heat and putting it all in the food processor.

I dump this into a pyrex container, and top it with tons of ground pepper. You can also add some cocoa powder and/or cinnamon. Once cooled, it goes in the freezer, so it can last a long time.

I occasionally chisel out a hunk of it using a knife, and put it in my lunch - since it's frozen, it keeps the rest of the lunch, usually a salad, cold, and by the time I'm ready to eat it, it's nicely thawed. It has a somewhat cheese like flavor to it, but it's really good.

Your beef heart is going off sometimes due to blood remaining in the tissue after slaughter. Blood will turn the meat very quickly, with the difficulties in removing blood from the heart and it's low value mean that greater care in purchase may be necessary. The black bits are blood BTW.

Blood remaining in the meat will turn it and make it taste odd. Sometimes during the slaughtering of the animal blood will speckle the heart meat, it could be due to the trauma or if the carcass was not bleed quickly. That is what the "black spots" are.