I have some problem with my blending. I load a STL file that give me triangles and  normals. I use those triangles and those normals to build and display an object.

I use two sided lighting and blending. So when I look at my object, I have some triangle that are not the same colors(darker).

any ideas?

Using blending for transparency effects need careful consideration of the order in which things are drawn (like back-to-front).
Do a search on the forum for blending and transparency and you’ll find lots of discussions on that topic.