NSColor transparency on black background?

In the background of my view, I draw a light blue color. And in the middle, i have a square box that is supposed to have an even lighter gray in it that has a 20% transparency. But for some reason the transparency is on top of a black background instea开发者_开发技巧d of a blue. I'm sorry If i'm not being clear.

You're probably using the wrong compositing mode to draw the smaller box. Show us your code.