If you have True Darkness installed alongside Immersive Portals, you will see the effect of the mod through a portal in a dimension where it's not supposed to be.
For example, if you have Dark Nether enabled and look through a portal to the Overworld, you will see it as pitch-black aside from light sources.
If you look through a portal to the Nether, dark areas will be visible as without the mod.

Examples:
In the Overworld, looking to the Nether:

In the Nether, looking to the Overworld: