An Israeli-led scientific study of Second Temple period phylacteries found near Qumran has found that they were not colored black, as is mandated by contemporary Jewish law. Neither of the two methods (soot/charcoal, and tannin with iron oxide) were used on these 2000-year-old relics.

https://www.jns.org/tefillin-were-not-dyed-black-2000-years-ago/