Making an image semi-transparent… Or is that semi-opaque?

[HavingTooMuchFun]

I decided to try doing a few rounds of color to alpha to see if it would do the weird thing it sometimes does, and it DID! In this first image, you can see that I have selected the RED blob, and brought up the color to alpha box… but the box is inexplicably showing that BLACK is selected, although it’s not, I did nothing in the alpha box to make it show black, there is no black in this image, and the previous selection was BLUE, not black. And here’s where it gets tricky: the alpha box is ALSO showing a checkerboard overlay on the red… NOT an all-gray checkerboard REPLACING the red, but an OVERLAY:



I click OK, and then I have this; note that the red blob has a checkerboard OVERLAY, rather than what color to alpha normally does, which would be replacing it with an all-gray checkerboard meaning that it is now transparent:



The red blob is now SHEER, as if it was a stained glass pane. Need more proof? Here is a boldly-patterned image:



And here is what I get if I copy the blob image over the black-and-white one… Note that the black lines show through the red, so the red blob is in fact SHEER, as if it was stained glass:



If you can tell me what happened here, so that I can reproduce it at will, I will be eternally grateful!