While in alpha mode it may look just fine, the result in the 1-bit transparency is, well look at it yoursefl.
Especially when you use magenta as transparent color for convenience. And no, masking off transparent colors doesn’t help since apparently blend mode samples colors no matter whether they’re in a masked area or not.
Related, but too minor to have it’s own topic: Apparently Blend Mode remembers the last color it was used on, so if you e.g. blended grass and then started to blend the sky to make nice clouds, suddenly your sky becomes green.