The whole concept of black can get weird if you let it. I know a guy - he's from Ethiopia - skin as black as can be - but he swears he's not black.
Black people, according to him, are from the Carribean. That's what black people means. Them.
He's not black.
He's actually a bit racist against black people, this fellow from Ethiopia.
In Rwanda one of the many reasons the Tutsi's got killed in genocide is because they considered the Hutu's blacks. Hutus, even though they had the majority of the political control in the country and the majority of the population, always resented the Tutsi for this among other things.
Generally in Africa any of the Bantu descended ethnicities are the "blacks", where the majority that got shipped to the Caribbeans and Brazil as slaves came from. You can see in this map how the main branches of Bantu ethnicities are spread.
In the horn of Africa, Somalis, Ethiopians, Djiboutians, Eritreans don't have many Bantu descendant ethnicities, that's why your friend denies being black. And furthermore Ethiopia divides their State lines by majority of ethnic groups, it's inconceivable for your co-worker to consider himself black.
It's not such a big deal in Ethiopia but in neighboring The Sudan that's what led
to the creation of South Sudan, Arab Sudanese majority hating on Nilo-Saharan - Bantu "black" folks.
Fascinating when you go down that rabbit hole. Bantu alone have over 600 ethnicities and 650 languages, Sudan alone over 500 ethnicities. Africa is cool like that, but their level of ignorance and hatred makes the West's racism pale in comparison.
Humans are very weak creatures that constantly need social validation and once in their circle feel the need to draw lines around it. We are animals.