Don't know off-hand... not sure I've seen that before.

Might be useful to know where the affected leaves are primarily found (top, middle, bottom, everywhere), what state you are located in (for clarification of temp, humidity & seasonal shifts) and if other dahlia plants are being treated/fertilized the same way without the dark splotches. Also, looking at the dark spots, I can't tell if the tissue is necrosed in that area or not... IOW, is the tissue sunken in or flush with the surrounding healthy green tissue?

Here's an interesting video on dahlia nutrient deficiencies that might at least help rule certain things out.

https://www.youtube.com/watch?v=qPVb7XnUMX4