Which Stack Exchange site is best for AutoCAD questions like using Blender for graphical design?
I don't see a site for AutoCAD.
For Blender, you can use this site: https://blender.stackexchange.com/
Currently there is no SE site specific for Autocad, but you can support the CAD website proposal in Area 51.
AutoCAD questions in general are on-topic for Engineering.SE. This also goes for other similar programs (ProE, Solid Works, Microstation, etc.)
CAD deserves its own page on Stack Exchange and it's your responsibility as a user to see it through.
The latest CAD Area 51 project is here but at 22 users as of January 27 2016, it is fizzling. Please join.
There is a reddit post on /r/cad (which is apparently a pretty active subreddit and may serve as a temporary alternative to a separate SE page) asking people to join the above petition. Upvote this CAD subreddit post to further support the effort here.
gis.stackexchange.com exists, cad.stackexchange.com deserves to exist too.
I agree with boulder_ruby, A Stack Exchange for CAD would be a big plus. Many online sites already exist for CAD. However, the Stack Exchange mechanism seems to generally work better. I am not working with CAD of late. However, back when I was doing AutoCAD, a Stack Exchange site would have been very useful. Same holds for SketchUp.