Sure you can, darker ink covers lighter ink. But do you not like the heart anymore ? You could always have the heart touched up and colored bright red again if you wanted. Otherwise a skilled cover up tattoo artist should be able to hide the heart.
I say: "Go for it girl!" :) Was the red heart a tattoo from before? Just please make sure that the black tattoo you're getting is not misspelled, or a stupid picture like a pile of poop lol. What are you planning on getting? Good luck!