If I Gaussian Blur white, I get white. No surprise.
If I then pipe this through a Mix node with any other color—say, yellow—the output is black.
I'm baffled. Why does this happen?
This is with the default scene settings, just switched to Cycles.
It also happens if I pass the blur through Separate RGBA and Combine RGBA first, but I've verified that each of the channels is at 1.000 (by sending them directly to the Composite node's image, which appears white). It doesn't work if I swap out the Blur node for an RGB node with pure white.
It does not happen if I check the "include alpha of second input" box on the mix node; in that case, the base color is used and the blur input is ignored.
Reproduced on Mac 10.10.3 and Linux Mint 17.2.