Can I use hook_block_info() in a theme?
I'm afraid you can't, no, it has to be in a module.

The array of blocks is built up in _block_render_blocks() using the module_invoke() function which, as its name suggests, only invoke hooks for modules and not themes.

If you haven't written a custom module before don't be afraid to get stuck in its pretty easy. I'd highly recommend the Module Developer's Guide which will tell you everything you need to know.