Melanin

Melanin was a biological pigment present in Bill Potts' skin. Bill cited the melanin in her skin as a reason why she shouldn't go out in 1814 London, due to the prevalence, in that time, of slavery.

Though she faced racism on that trip from one individual, Bill was overall surprised with how "black" London was, contrary to the whitewashing seen in history books. (TV: Thin Ice)