One of the things about blender that was frustrating for me when I last tried it ~3 years ago was the inability to use a different python REPL (ex: iPython) or easily import the blender python libraries in a non-blender python process. Can anyone say if this has gotten better?
Almost all of bpy is a simple generated wrapper around builtin C functions but you can (or could, dunno how well its been maintained?) build blender as a python module to import into CPython. Some experimental CMake setting IIRC.
