So to answer the question. The blue ridge is an effective screen because there is a limited number of passable gaps for military forces to cross. You can effectively block those gaps with a small amount of men against a superior force and create a bottleneck
It's sometimes hard for me to understand what the mountains in the eastern US look like because I live in the shadow of the coast range. Photos like this are a big help.
A few years ago, we drove from our home in Pennsylvania to Fairfax County, VA for an errand. Here is a photo that I have saved on my phone from our Virginia road trip.