Why did my blue-black ink get darker on the page?

You are watching iron-gall chemistry work: the ink goes down blue-grey, then iron compounds oxidize on contact with air and darken toward near-black over minutes to days. It is the intended behavior — the same reaction that made iron-gall the permanent record ink for a thousand years.

Modern iron-galls are pen-safe with basic hygiene: do not let them dry in the pen, and rinse on a schedule. The color shift is a feature you paid for.