When I create my own texture and add it to blocks of my level it looks wierd in the 3D window view. It has like dark areas. But when I load my level I have no problem. Where do I fix it?

--->I saw a post like this long time ago about a guy having the same problem as me but because I was not into texturing yet I did not pay attention to it. I tried to remember where I saw that post and did not work.