Clone stamp in darken mode not working as expected

I'm trying to remove a light halo that extends into both the blue and magenta areas. Following instructions at this website, with the clone stamp tool set to darken mode, I sampled the blue area (about 55 luminance) and painted the halo with a clone stamp brush large enough to cover the halo and a little of both the blue and magenta areas. 

The article says, "With Darken mode, however, only the pixels that are LIGHTER than the source pixels are replaced." So, I expected that with darken mode only pixels lighter than the 55 luminance blue source would be affected, but as you can see, it's darkening into the magenta, which is 40-42 luminance, producing a blue of 30 L. And for some reason, this unwanted darkening only extends 3-4 pixels into the magenta.

Any idea why it's affecting pixels darker than my 55 L source?

Is there a better way to remove a halo like this, that extends into two adjacent colors?

Thanks,
Russell