The OP prompted me to check on Shanhui Fan's group[1], which had done some sub-ambient radiative cooling in sunlight[2], and I stumbled on this recent work:
A dual-mode textile for human body radiative heating and cooling[3]
Unless you go out in the sun. But yes, if you're hot indoors, wearing black clothing can help by radiating more heat. (And white clothing can help in the sun by reflecting more incoming radiation.)
A dual-mode textile for human body radiative heating and cooling[3]
[1] https://web.stanford.edu/group/fan/ [2] https://web.stanford.edu/group/fan/publication/Goldstein_Nat... [3] https://web.stanford.edu/group/fan/publication/Hsu_ScienceAd...