Using Alpha with Material Nodes

I’m sort of poking my way through getting material nodes to do what I want, but I’m running into a problem.

I’m trying to combine a base layer (which will be edited and changed regularly, for baking in different colors) And a decal over the top of it, to give more detail.

At first, this solution seemed to be working fine for me, until I tried baking parts other than the one I made this particular material for. They were significantly lighter.

After a bit of poking, I found out why.
The lower symbol, on the black background, is actually alpha. I’m not sure why blender uses black, rather than the traditional checkerboard design to represent alpha. Perhaps I did something wrong in that particular material, although I don’t believe so.

The mix node is treating the black part of the texture (which should be alpha, and nonexistent) as a solid black patch of color, and darkening the base metal appropriately. This is not what I want. If you compare the upper input, with the finished output, the output is clearly darker.

I notice there’s an Alpha output on the material node, too. I have this feeling I should be doing something with that, but all my attempts at kicking it into working have failed thus far. Can anyone explain how to make the decal material’s alpha work properly, so it doesn’t have a solid black part darkening the base?

Try attaching the alpha of the decal material with the fac input of the mix node.

Already tried that. No use. This happens:

http://i103.photobucket.com/albums/m143/WarKirby/matnodesalpha2.png

Can you tell me, perhaps, if the black background is what alpha should look like, or does that indicate that I’ve made a mistake in the material?

Ohh, nevermind, I’ve fixed it. It was a matter of setting the base material of the decal to 0 alpha. Apparently, that doesn’t affect the texture layers on top of it…