All you need to know about the red heart is in the one OMG-no-they-didn't scene between Paul D and Beloved:
"Beloved." He said it, but she did not go. She moved closer with a footfall he didn't hear and he didn't hear the whisper that the flakes of rust made either as they fell away from the seams of his tobacco tin. So when the lid gave he didn't know it. What he knew was that when he reached the inside part he was saying, "Red heart. Red heart," over and over again. Softly and then so loud it woke Denver, then Paul D himself. "Red heart. Red heart. Red heart." (11.33)
It's pretty tempting to say that the red heart represents love, passion, life—something like that. And yeah, it kind of does mean all of those things, but it's also got a gothy feel to it, don't you think? Sure, sex with Beloved kind of reawakens Paul D's dormant emotional side, but he pretty much had to cross over to the dark side to get that feeling back.
Let's put it this way: if you believe that Beloved is a zombie-ish creature, Paul D basically has sex with a dead girl. And as he says "red heart" over and over again, he's more or less following the rhythm of sex (which is also probably why Denver wakes up from her spying spot). So the "red heart" may reaffirm life and all that jazz, but it can also only be experienced as an encounter with death and the past—both of which Beloved represents.
Which leads us to Sethe's own visions of red. Or actually, pink—pink flecks and chips on Beloved's headstone, to be exact.
You might notice that Sethe can't help but recall those pink flecks in the headstone every time she thinks of her dead baby girl. No small wonder, considering she was holding the baby when she saw "red baby blood" (3.107). The two—baby blood and pink-flecked headstone—go hand in hand. If the blood of a dying baby is vital, fresh and, therefore, bright red, it makes sense that the lighter shade of red—pink—comes to represent the trace of the dead baby, completely gone.
Plus, it's kind of morbid to have a pink headstone decorating baby Beloved's grave, don't you think? It's like a twisted version of a pink nursery for a living baby girl.
All in all: creepy.