I don't mean to ask "why blood," of course; we're all clear that the character is a vampire. And I don't mean to ask "why hidden" -- the core metaphor of that episode, and the fallback metaphor of most episodes, is vampirism as addiction, and so Nick behaves like an addict when that metaphor is in force. But why the chimney? Above the fireplace? What would rising heat do to wine-cut blood? Should it even still be liquid when he goes back for it?
Further, in "Dark Knight," Alyce finds the fireplace dusty, and wonders whether Nick has ever used it at all. When Nick first hid that bottle, did he expect the fireplace to be a safe hiding place because he intended never to use the fireplace? However, there was most certainly a fire in "Dark Knight," and then again at least for a moment in "For I Have Sinned." So what would that mean for the time the bottle has been hidden there before being pulled out in FtB? Would it necessarily have been planted since the last time the fire burned, or could it have been there since Nick first moved in? And would the carved mantle (cf. "Baby, Baby") be involved in some way?