First thing--- every deer I've tracked that's been heart shot has left nearly no blood, or even none at all. Keep in mind, with the heart damaged/destroyed, there's nothing to pressurize/push the blood. It becomes unpressurized/stagnant, which means it's not being forced out.
Second thing--- there are a ton of variables in whether blood will leak or not. Shot placement and angle are at the top of the list. Very solid vital hits can still produce horrid blood trails, whether with an ML or a rifle or a bow.
My buck this year was a less than ideal hit in the liver. Not at all what I wanted. When I backtrailed him the next day in daylight, there was a very substantial blood trail from where he laid down all the way back to the spot the arrow hit him. You'd never have expected it. Yet I shot one a few years ago in the snow (that freak Halloween snowfall) where the arrow passed just above the heart, and even in snow there was hardly any blood. Same arrows, same broadhead, minimal blood.
I shot one with my 62-cal flintlock that was DRT'ed. When I walked up to it, you'd think I'd scared it to death. Neither entrance nor exit could be found! I didn't find any actual holes till I peeled the hide. The angle at which I hit her was such that the hair was able to lay over the holes and cover them. Had she run, I'd imagine the blood trail would have been minimal until such point as there was enough soaked through the hair to start to drip, and she'd likely have been dead by then.
Shot one last January with the same 62-cal. X-ring hit behind the shoulder but she was quartering toward me and I didn't realize it. Ball went in tight to the back of the shoulder and right above the elbow. Came out behind the ribs on the far side. She started leaving blood about 10-15 feet from the hit (hair everywhere at the site of the hit), and left a reasonably good trail, sometimes a really substantial trail, for 200 yards till she died. I tracked her with my iPhone flashlight, if that's any indicator, lol.
It really does vary. I've found the most reliable hits for a good blood trail happen in a couple ribs back from the shoulder, and right about midway top to bottom. That seems to hit the most lung tissue, which seems to be a trigger or a good trail. And as soon as I've said that, a deer will prove it doesn't always work that way.