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/
AutoCAD questions in general are on-topic for Engineering.SE. This also goes for other similar programs (ProE, Solid Works, Microstation, etc.)
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.
Currently there is no SE site specific for Autocad, but you can support the CAD website proposal in Area 51.
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.