I would like to draw triangles (with the same Y - up direction - value), which ones may intersect each other. When I draw it with alpha value, I see darker blobs.
How can I resolve it?
I have a plane quad (in red colors), and above the quad a lot of randomly generated triangles with the same Y(up direction) value.
It looks more like a zfighting problem mixed with blending.
Disable depth test, to see if it gets uniformly dark.
To sort that out properly, you can use the stencil buffer when drawing your grey triangles.
Increment stencil on draw, and set the stencil test to =0 or something like that. So that each pixel of the screen will only be drawn once.