Without the complete botanical names of both trees it would be difficult to say if the blue spruce will work for you. I also found it confusing that you said you were planting it under another tree. Maybe you mean next to?
Blue spruce isn't a long lived tree and would be more prone to disease then a holly. Maybe you can post a picture?
I've also moved your question to the 'Trees' forum so you have a better chance of getting more answers.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.