This is becuase Legally Black people were equal with white people, there was no reason to document colour, this stopped when slavery was abolished. You have This documented in the US becuase Black people were second class.
As a family historian, can I say how very true this is? I just can't get over the difference in UK and US records. US censuses right up to 1940 (most recent available) have a specific column for "Colour or Race", in addition to those for Citizenship, Mother Tongue, etc.
UK censuses vv occasionally include comments added by the enumerator eg. "Helen Smith, a black woman", but there's no column for it.
The UK comment is racist, with the individual treating blackness as something different and worth commenting on. But it's not institutional racism as in the US, where race HAD to be recorded on EVERYTHING: censuses, military registers for potential call-up, birth/marriage/death records...
Dealing with the US records is eye-opening. And sickening. Though as an historian I recognise there's value to be extracted, because it makes the historic racism visible and also prevents modern researchers importing their own assumption that, say, the people they're studying are all non-black.