If the sections directly above some blocks are empty and not present in a chunk, then bluemap assumes a sunlight of 15 which is correct in the overworld, but wrong in the end and the nether dimensions. Resulting in weird lighting issues.

BlueMap Version: 1.5.4 - Minecraft 1.17

Fixed in 1.6.1

I'd like to add that these lighting issues also mess up the overview map

The light spots you see, are the light bugs.

When zoomed in, its supposed to look like this.

@SulpherStaer I am assuming you are using the Snapshot-Version (1.6.0)? That version has a different, known bug right now which causes this.
But that has nothing to do with this issue :)