This is not going to be as easy as you think it would be, because only the Unity Editor can load Blender files. When you import a Blender file, it is converted to Unity's internal format. This is the only format which the actual game engine can read. The built game does not include any Editor-specific program code, including the code which imports assets from 3rd party formats. That means when you want to allow the built game to read .blend files at runtime, you need to include your own parser for the .blend file format. And if you go that far, then you can just load any meta-data from the Blender file you want, for example in form of a text datablock created with the text editor.
But note that there are games which built thriving modding communities of people who use the Unity Personal Edition to create asset bundles which the base game can then load as mods. Kerbal Space Program, for example. This makes modding a lot easier than using Blender, because you can release a pack of handy editor scripts and other assets which help your modders to perform common tasks.
Another positive aspect of using Unity is that the full power of the Unity scripting system is at their disposal for adding new game features. But that does of course also give great potential for abuse.