Boolean Blunders: A heartfelt request.

Hello everyone! I am struggling with a model for print that… I just cannot seem to wrap my head around for whatever reason and I was hoping to glean knowledge from those more experienced than myself.
I am working on a mold of sorts involving a heart. (heartfelt request? Eh? Eh… okay.) It is going to require several layers and variants, and I just do not seem to be able to figure out a way to make one of them work. I have access to and decent experience with Zbrush, Blender, and Tinkercad, so any workarounds that are compatible with those would be super helpful.

http://3dprintboard.com/attachment.php?attachmentid=6724&d=1438058583&thumb=1

In the example above, you can see that I have the organ booleaned out of a solid block. But the trouble is, now I need a hallow version of /both filled areas/ with the empty spaced filled. Essentially reversing the polarities as it were. I am certain there is some sort of obvious process for this but for the life of me I keep just making a mess of the thing. Would anyone be able to shine some light on this dillema for me and potentially help me solve it? I would be deeply grateful.

Thank you,
Rykka.

I’m not able to see an image in your post. I think that’s specific to new users with under a certain number of posts, but I can’t remember.

By any chance are you talking about something like the boolean modifier in blender? I think you can make a hollow in an object by using the difference option in the modifier’s drop-down menu.