I actually know that blender is "hidden" in C:\Program Files (x86)\Steam\steamapps\common\Blender and I can tell Windows to open with the Blender.exe file in there but that will not count as opening Blender through Steam (I'm beat obsessed with the time that the application runs) the other option is to actually open the file within Blender, but if I want to open directly the file with a simple double click isn't the thing that I want, so if any body knows how to make to Steam relate the blend file extension to the application would help me.
On a second read ... I realise the formal post may not be the answer you are looking for. You may be referring to opening ".blend" files from the steam console where the steam application acts like a file browser? If that's what you are asking for then I am afraid it's impossible or at least I know the steam app is not design to be a file browser.
1. Open Up Steam
2. Go to "Library" tab
3. Go to "Games" List
4. At bottom left-hand corner click on "Add A Game"
5. Browser for the blender.exe linking to the steam application
6. Move your link to "Add to Favorite"
- Done! Now you can run blender from the bottom right-hand steam taskbar button!