I've written about Mykel Hawke on occasion here. I own both his Green Beret Survival Manual and his Special Forces Survival Handbook. Both have a wealth of information. I also used to like to watch him on Man, Woman, Wild.

One of the things he likes to say when it comes to wild berries at face value sounds really good:

If they are purple or blue, they are good for you.
If they are yellow or white, they probably ain't right.
If they are red, you have a 50/50 chance.
Let me introduce to you a blue berry that grows in abundance near my house:

This plant is known as Virginia Creeper or Parthenocissus Quinquefolia. It has bluish berries and red stems, and looks rather tasty. However according to NC State University these berries are poisonous!

Some symptoms of eating these berries if you are lucky are nausea, abdominal pain, bloody vomiting and diarrhea, dilated pupils, headache, sweating, weak pulse, drowsiness, twitching of face. If you are un-lucky you could just straight up die!

So be careful what you hear on television survival shows. This rule of thumb of Captain Hawke's might end up getting you killed. Remember, if you can't positively identify the plant, DO NOT EAT IT!