I was looking in an antique book of poetry 'The four winds of Eirinn' by Ethina Carberry and she has a poem called 'Mo Bhachaill cael-dubh' which she has translated as 'My Black Slender Boy' and the poem makes pretty clear she means Black, and not just dark-haired. It was quite a shocking poem to read, written around 1900 in Eire.