Thirty or more large black flies in one area, can this be associated with negative emotions and or evil?

No, it usually means there is something filthy around. Flies congregate around anything they can lay eggs on.

If you are in North America, this is the time of year to see large, slow moving flies called "Cluster Flies" begin hatching. Eggs were laid last fall and are just now beginning to hatch out. Likely you will see them congregating around food, water or sunlight (trying to get outside).

No, it is just something that happens. There is nothing evil about it, It is just nature and how they operate.